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/tokenizer/kinds.rs

@@ -40,6 +40,7 @@ pub enum TokenType {
     CLEANUP,
     THROW,
     LOCAL,
+    FLOW,
     STATIC,
     OUTBOX,
     NOT,

src/tokenizer/lex_ident.rs

@@ -46,6 +46,7 @@ impl NyashTokenizer {
             "cleanup" => TokenType::CLEANUP,
             "throw" => TokenType::THROW,
             "local" => TokenType::LOCAL,
+            "flow" => TokenType::FLOW,
             "static" => TokenType::STATIC,
             "outbox" => TokenType::OUTBOX,
             "not" => TokenType::NOT,
@@ -61,12 +62,13 @@ impl NyashTokenizer {
             _ => TokenType::IDENTIFIER(identifier.clone()),
         };
 
-        // Stage-3 gate: LOCAL/TRY/CATCH/THROW require NYASH_PARSER_STAGE3=1
+        // Stage-3 gate: LOCAL/FLOW/TRY/CATCH/THROW require NYASH_PARSER_STAGE3=1
         let stage3_enabled = crate::config::env::parser_stage3();
         if !stage3_enabled {
             let is_stage3 = matches!(
                 tok,
                 TokenType::LOCAL
+                    | TokenType::FLOW
                     | TokenType::TRY
                     | TokenType::CATCH
                     | TokenType::THROW
@@ -83,6 +85,7 @@ impl NyashTokenizer {
                 let is_stage3 = matches!(
                     tok,
                     TokenType::LOCAL
+                        | TokenType::FLOW
                         | TokenType::TRY
                         | TokenType::CATCH
                         | TokenType::THROW