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/boxes/cfg_navigator.hako

@@ -1,6 +1,6 @@
 // cfg_navigator.hako — CfgNavigatorBox（ブロックの先頭/末尾シーク）
 
-using "lang/src/shared/common/string_ops.hako" as StringOps
+using selfhost.shared.common.string_ops as StringOps
 using "lang/src/shared/json/json_cursor.hako" as JsonCursorBox
 
 static box CfgNavigatorBox {

lang/src/vm/boxes/mir_vm_m2.hako

@@ -1,7 +1,7 @@
 // mir_vm_m2.nyash — Ny製の最小MIR(JSON v0)実行器（M2: const/binop/ret）
 
 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 MirVmM2 {
   _str_to_int(s) { return StringHelpers.to_i64(s) }

lang/src/vm/boxes/mir_vm_min.hako

@@ -1,7 +1,7 @@
 // mir_vm_min.hako — Ny製の最小MIR(JSON v0)実行器（const/compare/copy/branch/jump/ret の最小）
 using "lang/src/vm/boxes/op_handlers.hako" as OpHandlersBox
 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/shared/json/utils/json_frag.hako" as JsonFragBox
 using "lang/src/shared/json/json_cursor.hako" as JsonCursorBox
 using "lang/src/vm/boxes/operator_box.hako" as OperatorBox

lang/src/vm/boxes/seam_inspector.hako

@@ -2,7 +2,7 @@
 // Usage: import and call report(text) or analyze_dump_file(path)
 
 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/shared/common/mini_vm_scan.hako" as MiniVmScan
 
 static box SeamInspector {