feat(stage-b): Add FLOW keyword support + fix Stage-3 keyword conflicts

## ✅ Fixed Issues

### 1. `local` keyword tokenization (commit 9aab64f7)
- Added Stage-3 gate for LOCAL/TRY/CATCH/THROW keywords
- LOCAL now only active when NYASH_PARSER_STAGE3=1

### 2. `env.local.get` keyword conflict
- File: `lang/src/compiler/entry/compiler_stageb.hako:21-23`
- Problem: `.local` in member access tokenized as `.LOCAL` keyword
- Fix: Commented out `env.local.get("HAKO_SOURCE")` line
- Fallback: Use `--source` argument (still functional)

### 3. `flow` keyword missing
- Added FLOW to TokenType enum (`src/tokenizer/kinds.rs`)
- Added "flow" → TokenType::FLOW mapping (`src/tokenizer/lex_ident.rs`)
- Added FLOW to Stage-3 gate (requires NYASH_PARSER_STAGE3=1)
- Added FLOW to parser statement dispatch (`src/parser/statements/mod.rs`)
- Added FLOW to declaration handler (`src/parser/statements/declarations.rs`)
- Updated box_declaration parser to accept BOX or FLOW (`src/parser/declarations/box_definition.rs`)
- Treat `flow FooBox {}` as syntactic sugar for `box FooBox {}`

### 4. Module namespace conversion
- Renamed `lang.compiler.builder.ssa.local` → `localvar` (avoid keyword)
- Renamed file `local.hako` → `local_ssa.hako`
- Converted 152 path-based using statements to namespace format
- Added 26+ entries to `nyash.toml` [modules] section

## ⚠️ Remaining Issues

### Stage-B selfhost compiler performance
- Stage-B compiler not producing output (hangs/times out after 10+ seconds)
- Excessive PHI debug output suggests compilation loop issue
- Needs investigation: infinite loop or N² algorithm in hako compiler

### Fallback JSON version mismatch
- Rust fallback (`--emit-mir-json`) emits MIR v1 JSON (schema_version: "1.0")
- Smoke tests expect MIR v0 JSON (`"version":0, "kind":"Program"`)
- stageb_helpers.sh fallback needs adjustment

## Test Status
- Parse errors: FIXED ✅
- Keyword conflicts: FIXED ✅
- Stage-B smoke tests: STILL FAILING ❌ (performance issue)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/runner/child_env.rs

@@ -19,3 +19,21 @@ pub fn post_run_exit_if_oob_strict_triggered() -> ! {
     std::process::exit(0)
 }
 
+/// Apply a consistent child environment for selfhost/core wrapper executions.
+/// - Forces JSON-only quiet pipe
+/// - Disables plugins to avoid host-side side effects
+/// - Disables file-based using resolution (namespace-first policy)
+/// - Skips nyash.toml env injection to reduce drift
+pub fn apply_core_wrapper_env(cmd: &mut std::process::Command) {
+    // Remove noisy or recursive toggles
+    cmd.env_remove("NYASH_USE_NY_COMPILER");
+    cmd.env_remove("NYASH_CLI_VERBOSE");
+    // Enforce quiet JSON capture
+    cmd.env("NYASH_JSON_ONLY", "1");
+    // Restrict environment to avoid plugin/using drift
+    cmd.env("NYASH_DISABLE_PLUGINS", "1");
+    cmd.env("NYASH_SKIP_TOML_ENV", "1");
+    cmd.env("NYASH_USING_AST", "0");
+    cmd.env("NYASH_ALLOW_USING_FILE", "0");
+    cmd.env("HAKO_ALLOW_USING_FILE", "0");
+}

src/runner/dispatch.rs

@@ -37,7 +37,7 @@ pub(crate) fn execute_file_with_backend(runner: &NyashRunner, filename: &str) {
                 return;
             }
             Err(e) => {
-                eprintln!("❌ Direct bridge parse error: {}", e);
+                eprintln!("❌ Direct bridge parse error in {}: {}", filename, e);
                 process::exit(1);
             }
         }
@@ -56,7 +56,7 @@ pub(crate) fn execute_file_with_backend(runner: &NyashRunner, filename: &str) {
         let ast = match NyashParser::parse_from_string(&code) {
             Ok(ast) => ast,
             Err(e) => {
-                eprintln!("❌ Parse error: {}", e);
+                eprintln!("❌ Parse error in {}: {}", filename, e);
                 process::exit(1);
             }
         };
@@ -79,7 +79,7 @@ pub(crate) fn execute_file_with_backend(runner: &NyashRunner, filename: &str) {
         };
         let ast = match NyashParser::parse_from_string(&code) {
             Ok(ast) => ast,
-            Err(e) => { eprintln!("❌ Parse error: {}", e); process::exit(1); }
+            Err(e) => { eprintln!("❌ Parse error in {}: {}", filename, e); process::exit(1); }
         };
         let expanded = if crate::r#macro::enabled() {
             let a = crate::r#macro::maybe_expand_and_dump(&ast, false);

src/runner/modes/common.rs

@@ -114,7 +114,7 @@ impl NyashRunner {
                     ast
                 }
                 Err(e) => {
-                    eprintln!("❌ Parse error: {}", e);
+                    eprintln!("❌ Parse error in {}: {}", filename, e);
                     process::exit(1);
                 }
             };

src/runner/modes/common_util/resolve/strip.rs

@@ -72,7 +72,9 @@ pub fn collect_using_and_strip(
                 // can organize their modules via file paths.
                 if (prod || !crate::config::env::allow_using_file()) && !inside_pkg {
                     return Err(format!(
-                        "using: file paths are disallowed in this profile. Add it to nyash.toml [using] (packages/aliases) and reference by name: {}",
+                        "{}:{}: using: file paths are disallowed in this profile. Add it to nyash.toml [using] (packages/aliases) and reference by name: {}",
+                        filename,
+                        line_no,
                         target
                     ));
                 }
@@ -220,7 +222,9 @@ pub fn collect_using_and_strip(
                     }
                 } else {
                     return Err(format!(
-                        "using: '{}' not found in nyash.toml [using]. Define a package or alias and use its name (prod profile)",
+                        "{}:{}: using: '{}' not found in nyash.toml [using]. Define a package or alias and use its name (prod profile)",
+                        filename,
+                        line_no,
                         target
                     ));
                 }
@@ -311,7 +315,7 @@ pub fn collect_using_and_strip(
                             prelude_paths.push(path_str);
                         }
                     }
-                    Err(e) => return Err(format!("using: {}", e)),
+                    Err(e) => return Err(format!("{}:{}: using: {}", filename, line_no, e)),
                 }
             }
             continue;

src/runner/modes/common_util/selfhost/child.rs

@@ -17,6 +17,8 @@ pub fn run_ny_program_capture_json(
 ) -> Option<String> {
     use std::process::Command;
     let mut cmd = Command::new(exe);
+    // Apply consistent child env to avoid plugin/using drift
+    crate::runner::child_env::apply_core_wrapper_env(&mut cmd);
     cmd.arg("--backend").arg("vm").arg(program);
     for a in extra_args {
         cmd.arg(a);

src/runner/modes/cranelift.rs

@@ -13,7 +13,7 @@ impl NyashRunner {
         // Parse → AST
         let ast = match NyashParser::parse_from_string(&code) {
             Ok(ast) => ast,
-            Err(e) => { eprintln!("❌ Parse error: {}", e); process::exit(1); }
+            Err(e) => { eprintln!("❌ Parse error in {}: {}", filename, e); process::exit(1); }
         };
         let ast = crate::r#macro::maybe_expand_and_dump(&ast, false);
         // AST → MIR

src/runner/modes/llvm.rs

@@ -58,7 +58,7 @@ impl NyashRunner {
         let main_ast = match NyashParser::parse_from_string(code_ref) {
             Ok(ast) => ast,
             Err(e) => {
-                eprintln!("❌ Parse error: {}", e);
+                eprintln!("❌ Parse error in {}: {}", filename, e);
                 process::exit(1);
             }
         };

src/runner/modes/mir.rs

@@ -21,7 +21,7 @@ impl NyashRunner {
         let ast = match NyashParser::parse_from_string(&code) {
             Ok(ast) => ast,
             Err(e) => {
-                eprintln!("❌ Parse error: {}", e);
+                eprintln!("❌ Parse error in {}: {}", filename, e);
                 process::exit(1);
             }
         };

src/runner/modes/mir_interpreter.rs

@@ -15,7 +15,7 @@ impl NyashRunner {
         // Parse to AST
         let ast = match NyashParser::parse_from_string(&code) {
             Ok(ast) => ast,
-            Err(e) => { eprintln!("❌ Parse error: {}", e); process::exit(1); }
+            Err(e) => { eprintln!("❌ Parse error in {}: {}", filename, e); process::exit(1); }
         };
         let ast = crate::r#macro::maybe_expand_and_dump(&ast, false);
 

src/runner/modes/pyvm.rs

@@ -82,7 +82,7 @@ pub fn execute_pyvm_only(runner: &NyashRunner, filename: &str) {
     let ast = match NyashParser::parse_from_string(&code) {
         Ok(ast) => ast,
         Err(e) => {
-            eprintln!("❌ Parse error: {}", e);
+            eprintln!("❌ Parse error in {}: {}", filename, e);
             process::exit(1);
         }
     };

src/runner/modes/vm.rs

@@ -154,7 +154,8 @@ impl NyashRunner {
         let main_ast = match NyashParser::parse_from_string(code_ref) {
             Ok(ast) => ast,
             Err(e) => {
-                eprintln!("❌ Parse error: {}", e);
+                eprintln!("❌ Parse error in main source ({}): {}",
+                         cfg.file.as_ref().map(|s| s.as_str()).unwrap_or("<stdin>"), e);
                 if std::env::var("NYASH_STRIP_DEBUG").ok().as_deref() == Some("1") {
                     eprintln!("[vm-debug] Parse failed for main source");
                     eprintln!("[vm-debug] Line 15-25 of source:");

src/runner/modes/vm_fallback.rs

@@ -58,7 +58,7 @@ impl NyashRunner {
         let main_ast = match NyashParser::parse_from_string(&code2) {
             Ok(ast) => ast,
             Err(e) => {
-                eprintln!("❌ Parse error: {}", e);
+                eprintln!("❌ Parse error in {}: {}", filename, e);
                 process::exit(1);
             }
         };

src/runner/modes/wasm.rs

@@ -17,7 +17,7 @@ impl NyashRunner {
         // Parse to AST
         let ast = match NyashParser::parse_from_string(&code) {
             Ok(ast) => ast,
-            Err(e) => { eprintln!("❌ Parse error: {}", e); process::exit(1); }
+            Err(e) => { eprintln!("❌ Parse error in {}: {}", filename, e); process::exit(1); }
         };
         let ast = crate::r#macro::maybe_expand_and_dump(&ast, false);
 

src/runner/selfhost.rs

@@ -164,8 +164,18 @@ impl NyashRunner {
                     parser_prog,
                     timeout_ms,
                     &extra,
-                    &["NYASH_USE_NY_COMPILER", "NYASH_CLI_VERBOSE"],
-                    &[("NYASH_JSON_ONLY", "1")],
+                    &[
+                        "NYASH_USE_NY_COMPILER",
+                        "NYASH_CLI_VERBOSE",
+                    ],
+                    &[
+                        ("NYASH_JSON_ONLY", "1"),
+                        ("NYASH_DISABLE_PLUGINS", "1"),
+                        ("NYASH_SKIP_TOML_ENV", "1"),
+                        ("NYASH_USING_AST", "0"),
+                        ("NYASH_ALLOW_USING_FILE", "0"),
+                        ("HAKO_ALLOW_USING_FILE", "0"),
+                    ],
                 ) {
                     match json::parse_json_v0_line(&line) {
                         Ok(module) => {
@@ -360,9 +370,7 @@ impl NyashRunner {
                 .unwrap_or_else(|_| std::path::PathBuf::from("target/release/nyash"));
             let mut cmd = std::process::Command::new(exe);
             cmd.arg("--backend").arg("vm").arg(&inline_path);
-            cmd.env_remove("NYASH_USE_NY_COMPILER");
-            cmd.env_remove("NYASH_CLI_VERBOSE");
-            cmd.env("NYASH_JSON_ONLY", "1");
+            crate::runner::child_env::apply_core_wrapper_env(&mut cmd);
             let timeout_ms: u64 = std::env::var("NYASH_NY_COMPILER_TIMEOUT_MS")
                 .ok()
                 .and_then(|s| s.parse().ok())