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/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())