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/shared/common/box_helpers.hako

@@ -2,7 +2,7 @@
 // 目的: ArrayBox/MapBox の安全な操作を統一的に提供
 // 削減: 7ファイル × 2パターン = 14重複 → 1箇所に集約
 
-using "lang/src/shared/common/string_helpers.hako" as StringHelpers
+using selfhost.shared.common.string_helpers as StringHelpers
 
 static box BoxHelpers {
   // ArrayBox.size/1 の結果unwrap (MapBox-wrapped integer対応)

lang/src/shared/common/mini_vm_scan.hako

@@ -1,7 +1,7 @@
 // Mini-VM scanning and numeric helpers
 // Delegation Policy: all scanning primitives route to JsonCursorBox to avoid divergence.
-using "lang/src/shared/common/string_helpers.hako" as StringHelpers
-using "lang/src/shared/json/json_cursor.hako" as JsonCursorBox
+using selfhost.shared.common.string_helpers as StringHelpers
+using selfhost.shared.json.core.json_cursor as JsonCursorBox
 
 static box MiniVmScan {
   // helper: find needle from position pos (escape-aware where needed)

lang/src/shared/json/core/json_scan.hako

@@ -1,8 +1,8 @@
 // json_scan.hako — JsonScanBox
 // Escape-aware JSON structure scanning helpers.
 
-using "lang/src/shared/json/core/string_scan.hako" as StringScanBox
-using "lang/src/shared/common/string_helpers.hako" as StringHelpers
+using selfhost.shared.json.core.string_scan as StringScanBox
+using selfhost.shared.common.string_helpers as StringHelpers
 
 static box JsonScanBox {
   // minimal string→int (delegate to shared helper)

lang/src/shared/json/core/string_scan.hako

@@ -1,7 +1,7 @@
 // string_scan.hako — StringScanBox
 // Escape-aware string scanning helpers for JSON/text processing.
 
-using "lang/src/shared/common/string_ops.hako" as StringOps
+using selfhost.shared.common.string_ops as StringOps
 
 static box StringScanBox {
   // Return the single-character string at index i (empty if out of bounds)

lang/src/shared/json/json_cursor.hako

@@ -2,9 +2,9 @@
 // Responsibility: provide a minimal, escape-aware scanning facade used by JsonFragBox
 // Delegates to StringOps, StringScanBox and JsonScanBox.
 
-using "lang/src/shared/common/string_ops.hako" as StringOps
-using "lang/src/shared/json/core/string_scan.hako" as StringScanBox
-using "lang/src/shared/json/core/json_scan.hako" as JsonScanBox
+using selfhost.shared.common.string_ops as StringOps
+using selfhost.shared.json.core.string_scan as StringScanBox
+using selfhost.shared.json.core.json_scan as JsonScanBox
 
 static box JsonCursorBox {
   index_of_from(hay, needle, pos) {

lang/src/shared/json/json_utils.hako

@@ -1,7 +1,7 @@
 // JsonUtilsBox — JSON読み取りユーティリティの共通箱
 // 責務: JSON値抽出・JSON構造パース・トップレベル配列分割
 // Extracted from JsonProgramBox (480行 → 345行, 削減 ~135行)
-using "lang/src/shared/common/string_helpers.hako" as StringHelpers
+using selfhost.shared.common.string_helpers as StringHelpers
 
 static box JsonUtilsBox {
   // Extract JSON value by key (returns value with '@' + end position marker)

lang/src/shared/json/mir_builder2.hako

@@ -1,5 +1,5 @@
 // mir_builder2.hako — Instance builder（引数渡しバグ回避のため、非staticで内部状態を保持）
-using "lang/src/shared/common/string_helpers.hako" as StringHelpers
+using selfhost.shared.common.string_helpers as StringHelpers
 using lang.compiler.emit.common.json_emit as JsonEmitBox
 using lang.compiler.emit.common.mir_emit as MirEmitBox
 using lang.compiler.emit.common.header_emit as HeaderEmitBox

lang/src/shared/json/mir_builder_min.hako

@@ -1,13 +1,13 @@
 // mir_builder_min.nyash — Minimal MIR(JSON v0) builder for selfhost tests
 // Scope: selfhost only (apps/selfhost/...); no core/runtime changes.
 
-using "lang/src/shared/common/string_helpers.hako" as StringHelpers
-using "lang/src/shared/common/box_helpers.hako" as BoxHelpers
+using selfhost.shared.common.string_helpers as StringHelpers
+using selfhost.shared.common.box_helpers as BoxHelpers
 using lang.compiler.emit.common.json_emit as JsonEmitBox
 using lang.compiler.emit.common.mir_emit as MirEmitBox
 using lang.compiler.emit.common.call_emit as CallEmitBox
 using lang.compiler.emit.common.newbox_emit as NewBoxEmitBox
-using "lang/src/shared/json/json_inst_encode_box.hako" as JsonInstEncodeBox
+using selfhost.shared.json.json_inst_encode_box as JsonInstEncodeBox
 
 box MirJsonBuilderMin {
   buf: StringBox

lang/src/shared/json/mir_v1_adapter.hako

@@ -1,7 +1,7 @@
 // mir_v1_adapter.nyash — Minimal JSON v1 (mir_call) to v0 adapter for selfhost
 // Scope: selfhost only. Transforms op:"mir_call" into legacy v0 ops (call/boxcall/newbox).
 
-using "lang/src/shared/json/json_cursor.hako" as JsonCursorBox
+using selfhost.shared.json.core.json_cursor as JsonCursorBox
 
 static box MirJsonV1Adapter {
   // Delegate to JsonCursorBox (escape-aware implementations, fixes 2 escape bugs)

lang/src/shared/json/utils/json_frag.hako

@@ -2,8 +2,8 @@
 // 責務: 文字列JSONから key:int / key:str を簡便に取り出す。
 // 非責務: 実行・評価（構造検査やVM実行は他箱に委譲）。
 
-using "lang/src/shared/json/json_cursor.hako" as JsonCursorBox
-using "lang/src/shared/common/string_helpers.hako" as StringHelpers
+using selfhost.shared.json.core.json_cursor as JsonCursorBox
+using selfhost.shared.common.string_helpers as StringHelpers
 
 static box JsonFragBox {
   // 基本ヘルパ

lang/src/shared/mir/block_builder_box.hako

@@ -1,9 +1,9 @@
 // selfhost/shared/mir/block_builder_box.hako
 // BlockBuilderBox — small helpers to assemble P1 shapes
 
-using "lang/src/shared/mir/mir_schema_box.hako" as MirSchemaBox
-using "lang/src/shared/mir/loop_form_box.hako" as LoopFormBox
-using "lang/src/shared/common/string_helpers.hako" as StringHelpers
+using selfhost.shared.mir.schema as MirSchemaBox
+using selfhost.shared.mir.loopform as LoopFormBox
+using selfhost.shared.common.string_helpers as StringHelpers
 
 static box BlockBuilderBox {
 

lang/src/shared/mir/json_emit_box.hako

@@ -1,8 +1,8 @@
 // selfhost/shared/mir/json_emit_box.hako
 // JsonEmitBox — Gate C JSON emitter (schema_version 1.0, numbers unwrapped)
 
-using "lang/src/shared/common/string_helpers.hako" as StringHelpers
-using "lang/src/shared/common/box_helpers.hako" as BoxHelpers
+using selfhost.shared.common.string_helpers as StringHelpers
+using selfhost.shared.common.box_helpers as BoxHelpers
 
 static box JsonEmitBox {
   _expect_map(val, context) {

lang/src/shared/mir/loop_form_box.hako

@@ -1,7 +1,7 @@
 // selfhost/shared/mir/loop_form_box.hako
 // LoopFormBox — minimal loop structure builder (P2: continue/break snapshots + Exit PHI)
 
-using "lang/src/shared/mir/mir_schema_box.hako" as MirSchemaBox
+using selfhost.shared.mir.schema as MirSchemaBox
 
 static box LoopFormBox {
 

lang/src/shared/mir/mir_io_box.hako

@@ -9,15 +9,15 @@
 
 using "lang/src/vm/boxes/result_box.hako" as Result
 using "lang/src/vm/boxes/result_helpers.hako" as ResultHelpers
-using "lang/src/shared/json/json_cursor.hako" as JsonCursorBox
+using selfhost.shared.json.core.json_cursor as JsonCursorBox
 using "lang/src/shared/json/json_canonical_box.hako" as JsonCanonicalBox
 using "lang/src/vm/hakorune-vm/function_locator.hako" as FunctionLocatorBox
 using "lang/src/vm/hakorune-vm/blocks_locator.hako" as BlocksLocatorBox
 using "lang/src/vm/hakorune-vm/instrs_locator.hako" as InstrsLocatorBox
 using "lang/src/vm/hakorune-vm/backward_object_scanner.hako" as BackwardObjectScannerBox
 using "lang/src/vm/hakorune-vm/block_iterator.hako" as BlockIteratorBox
-using "lang/src/shared/common/string_helpers.hako" as StringHelpers
-using "lang/src/shared/common/box_helpers.hako" as BoxHelpers
+using selfhost.shared.common.string_helpers as StringHelpers
+using selfhost.shared.common.box_helpers as BoxHelpers
 
 static box MirIoBox {
   // Internal: provider gate (yyjson)

lang/src/shared/mir/mir_schema_box.hako

@@ -1,7 +1,7 @@
 // selfhost/shared/mir/mir_schema_box.hako
 // MirSchemaBox — minimal MIR(JSON v0) constructors (P1 scope)
 
-using "lang/src/shared/common/string_helpers.hako" as StringHelpers
+using selfhost.shared.common.string_helpers as StringHelpers
 
 static box MirSchemaBox {
   _expect_map(val, context) {