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>

lang/src/compiler/entry/compiler.hako

@@ -2,7 +2,7 @@
 // - When invoked with --min-json, emit minimal Program JSON v0 to stdout
 // - Otherwise, act as a silent placeholder (return 0)
 
-using "lang/src/compiler/entry/compiler_stageb.hako" as StageBMain
+using lang.compiler.entry.compiler_stageb as StageBMain
 
 static box Main {
   _parse_signed_int(raw) {

lang/src/compiler/entry/compiler_stageb.hako

@@ -18,7 +18,8 @@ static box StageBMain {
         i = i + 1
       }
     }
-    if src == null { src = env.local.get("HAKO_SOURCE") }
+    // Skip env.local.get check - Stage-3 keyword conflict
+    // Original: if src == null { src = env.local.get("HAKO_SOURCE") }
     if src == null { src = "return 0" }
 
     // 2) Stage‑3 acceptance default ON for selfhost (env may turn off; keep tolerant here)
@@ -72,7 +73,7 @@ static box StageBMain {
     local prefer = 1
     local out = FlowEntryBox.emit_v0_from_ast_with_context(ast_json, prefer, usings_json, null, externs_json)
     if out == null || out == "" { out = FlowEntryBox.emit_v0_from_ast(ast_json, prefer) }
-    if out == null || out == "" { out = "{\"version\":0,\"kind\":\"Program\",\"body\":[{\"type\":\"Return\",\"expr\":{\"type\":\"Int\",\"value\":0}}]}" }
+    // TTL OFF: do not fallback to dummy JSON when emit fails. Empty output surfaces failure to caller.
     print(out)
     return 0
   }