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/using/resolver.rs

@@ -15,11 +15,16 @@ pub fn populate_from_toml(
     packages: &mut HashMap<String, UsingPackage>,
 ) -> Result<UsingPolicy, UsingError> {
     let mut policy = UsingPolicy::default();
+    // Prefer CWD nyash.toml; if missing, honor NYASH_ROOT/nyash.toml for tools that run from subdirs
     let path = std::path::Path::new("nyash.toml");
-    if !path.exists() {
+    let text = if path.exists() {
+        std::fs::read_to_string(path)
+    } else if let Ok(root) = std::env::var("NYASH_ROOT") {
+        let alt = std::path::Path::new(&root).join("nyash.toml");
+        if alt.exists() { std::fs::read_to_string(alt) } else { return Ok(policy); }
+    } else {
         return Ok(policy);
     }
-    let text = std::fs::read_to_string(path)
         .map_err(|e| UsingError::ReadToml(e.to_string()))?;
     let doc = toml::from_str::<toml::Value>(&text)
         .map_err(|e| UsingError::ParseToml(e.to_string()))?;