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/vm/hakorune-vm/args_extractor.hako

@@ -1,7 +1,7 @@
 // ArgsExtractorBox - Extract and load arguments from MirCall JSON
 // Single Responsibility: Parse args array, load values from registers
 
-using "lang/src/shared/common/string_ops.hako" as StringOps
+using selfhost.shared.common.string_ops as StringOps
 using "lang/src/shared/common/string_helpers.hako" as StringHelpers
 using "lang/src/vm/hakorune-vm/value_manager.hako" as ValueManagerBox
 

lang/src/vm/hakorune-vm/binop_handler.hako

@@ -2,7 +2,7 @@
 // Handles: %dst = %lhs op_kind %rhs (Add/Sub/Mul/Div/Mod)
 
 using "lang/src/shared/common/string_helpers.hako" as StringHelpers
-using "lang/src/shared/common/string_ops.hako" as StringOps
+using selfhost.shared.common.string_ops as StringOps
 using "lang/src/vm/boxes/result_box.hako" as Result
 using "lang/src/vm/hakorune-vm/value_manager.hako" as ValueManagerBox
 using "lang/src/vm/hakorune-vm/reg_guard.hako" as RegGuardBox

lang/src/vm/hakorune-vm/block_mapper.hako

@@ -3,7 +3,7 @@
 
 using "lang/src/shared/json/json_cursor.hako" as JsonCursorBox
 using "lang/src/shared/common/string_helpers.hako" as StringHelpers
-using "lang/src/shared/common/string_ops.hako" as StringOps
+using selfhost.shared.common.string_ops as StringOps
 using "lang/src/vm/boxes/result_box.hako" as Result
 using "lang/src/vm/hakorune-vm/function_locator.hako" as FunctionLocatorBox
 using "lang/src/vm/hakorune-vm/blocks_locator.hako" as BlocksLocatorBox

lang/src/vm/hakorune-vm/callee_parser.hako

@@ -1,7 +1,7 @@
 // CalleeParserBox - Extract callee information from MirCall JSON
 // Single Responsibility: Parse callee field and extract type/name
 
-using "lang/src/shared/common/string_ops.hako" as StringOps
+using selfhost.shared.common.string_ops as StringOps
 using "lang/src/shared/common/string_helpers.hako" as StringHelpers
 using "lang/src/vm/hakorune-vm/json_field_extractor.hako" as JsonFieldExtractor
 

lang/src/vm/hakorune-vm/closure_call_handler.hako

@@ -3,7 +3,7 @@
 // Note: Full closure calling via Callee::Value is Phase 4 Day 16
 
 using "lang/src/vm/boxes/result_box.hako" as Result
-using "lang/src/shared/common/string_ops.hako" as StringOps
+using selfhost.shared.common.string_ops as StringOps
 using "lang/src/shared/common/string_helpers.hako" as StringHelpers
 using "lang/src/shared/json/json_cursor.hako" as JsonCursorBox
 

lang/src/vm/hakorune-vm/const_handler.hako

@@ -2,7 +2,7 @@
 // Handles: %dst = const value
 
 using "lang/src/shared/common/string_helpers.hako" as StringHelpers
-using "lang/src/shared/common/string_ops.hako" as StringOps
+using selfhost.shared.common.string_ops as StringOps
 using "lang/src/vm/boxes/result_box.hako" as Result
 using "lang/src/vm/hakorune-vm/value_manager.hako" as ValueManagerBox
 using "lang/src/vm/hakorune-vm/json_field_extractor.hako" as JsonFieldExtractor

lang/src/vm/hakorune-vm/hakorune_vm_core.hako

@@ -4,7 +4,7 @@
 // Phase 1 Day 3: 制御フロー実装（Branch/Jump/Phi）- 箱化モジュール化
 using "lang/src/shared/json/json_cursor.hako" as JsonCursorBox
 using "lang/src/shared/common/string_helpers.hako" as StringHelpers
-using "lang/src/shared/common/string_ops.hako" as StringOps
+using selfhost.shared.common.string_ops as StringOps
 using "lang/src/vm/hakorune-vm/json_scan_guard.hako" as JsonScanGuardBox
 using "lang/src/vm/boxes/result_box.hako" as Result
 // Phase 1 Day 3: 箱化モジュール化

lang/src/vm/hakorune-vm/instruction_dispatcher.hako

@@ -1,7 +1,7 @@
 // InstructionDispatcherBox - Dispatch instructions to specific handlers
 // Single Responsibility: Extract op field and route to correct handler
 
-using "lang/src/shared/common/string_ops.hako" as StringOps
+using selfhost.shared.common.string_ops as StringOps
 using "lang/src/vm/boxes/result_box.hako" as Result
 using "lang/src/vm/hakorune-vm/json_field_extractor.hako" as JsonFieldExtractor
 using "lang/src/vm/hakorune-vm/value_manager.hako" as ValueManagerBox

lang/src/vm/hakorune-vm/json_field_extractor.hako

@@ -2,7 +2,7 @@
 // Centralizes JSON field parsing logic
 
 using "lang/src/shared/common/string_helpers.hako" as StringHelpers
-using "lang/src/shared/common/string_ops.hako" as StringOps
+using selfhost.shared.common.string_ops as StringOps
 
 static box JsonFieldExtractor {
   // Extract integer field from JSON

lang/src/vm/hakorune-vm/newbox_handler.hako

@@ -4,7 +4,7 @@
 using "lang/src/vm/boxes/result_box.hako" as Result
 using "lang/src/vm/hakorune-vm/value_manager.hako" as ValueManagerBox
 using "lang/src/vm/hakorune-vm/json_field_extractor.hako" as JsonFieldExtractor
-using "lang/src/shared/common/string_ops.hako" as StringOps
+using selfhost.shared.common.string_ops as StringOps
 using "lang/src/vm/gc/gc_runtime.hako" as GcRuntime
 
 static box NewBoxHandlerBox {

lang/src/vm/hakorune-vm/phi_handler.hako

@@ -3,7 +3,7 @@
 
 using "lang/src/shared/json/json_cursor.hako" as JsonCursorBox
 using "lang/src/shared/common/string_helpers.hako" as StringHelpers
-using "lang/src/shared/common/string_ops.hako" as StringOps
+using selfhost.shared.common.string_ops as StringOps
 using "lang/src/vm/boxes/result_box.hako" as Result
 using "lang/src/vm/hakorune-vm/core_bridge_ops.hako" as CoreBridgeOps
 

lang/src/vm/hakorune-vm/terminator_handler.hako

@@ -2,7 +2,7 @@
 // Strategy: 箱化モジュール化 - Ret/Jump/Branch を分離
 
 using "lang/src/shared/common/string_helpers.hako" as StringHelpers
-using "lang/src/shared/common/string_ops.hako" as StringOps
+using selfhost.shared.common.string_ops as StringOps
 using "lang/src/vm/boxes/result_box.hako" as Result
 using "lang/src/vm/hakorune-vm/instrs_locator.hako" as InstrsLocatorBox
 using "lang/src/vm/hakorune-vm/backward_object_scanner.hako" as BackwardObjectScannerBox

lang/src/vm/hakorune-vm/unaryop_handler.hako

@@ -2,7 +2,7 @@
 // Handles: %dst = op_kind %operand (Neg/Not/BitNot)
 
 using "lang/src/shared/common/string_helpers.hako" as StringHelpers
-using "lang/src/shared/common/string_ops.hako" as StringOps
+using selfhost.shared.common.string_ops as StringOps
 using "lang/src/vm/boxes/result_box.hako" as Result
 using "lang/src/vm/hakorune-vm/value_manager.hako" as ValueManagerBox
 using "lang/src/vm/hakorune-vm/reg_guard.hako" as RegGuardBox