docs: Phase 195 Pattern 3 extension design (multi-carrier support)

Phase 195 design document for P3 (If-Else PHI) multi-carrier expansion:
- Goal: Handle 2-3 carriers in if-else branches simultaneously
- Scope: if-complete multi-carrier updates (flag+buffer, sum+count patterns)

Task breakdown:
- 195-1: Loop inventory (_parse_string simple, if-sum patterns)
- 195-2: LoopUpdateSummary/CarrierInfo design (multi-carrier support)
- 195-3: Pattern3 lowerer design (PhiGroup vs ExitLine extension)
- 195-4: Implementation scope (2-3 carriers, existing UpdateKind range)
- 195-5: Documentation updates (CURRENT_TASK.md + overview)

Design decisions:
- ✅ ExitLine extension (no PhiGroupBox - YAGNI principle)
- ✅ Both-branch carrier definition check (unchanged = use previous value)
- ❌ No ConditionEnv expansion (Phase 200+ deferred)
- ❌ LoopBodyLocal + MethodCall mix deferred to Phase 195+

Target loops:
1. JsonParser _parse_string (escaped flag + buffer)
   - Carrier 1: escaped (BoolFlag) - conditional flag
   - Carrier 2: buffer (StringAppend) - else-only update
2. selfhost if-sum (sum + count)
   - Carrier 1: sum (NumberAccumulation) - then-only update
   - Carrier 2: count (CounterLike) - then-only update

Test cases designed:
- phase195_flag_buffer.hako (BoolFlag + StringAppend)
- phase195_sum_count.hako (NumberAccumulation + CounterLike)

Expected outcome:
- phase195-pattern3-extension-design.md (complete design spec)
- Clear implementation scope for Phase 195-impl
- Path to 40% → 60% JsonParser coverage

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

docs/development/current/main/joinir-architecture-overview.md

@@ -509,12 +509,28 @@ JoinIR は Rust 側だけでなく、将来的に .hako selfhost コンパイラ
    - テスト: phase192_normalization_demo.hako → 123 ✅
    - 制約: MethodCall を含む init 式は Phase 193 で対応予定
 
-3. **MethodCall を含む init 式の対応（Phase 193）**
-   - `local digit = digits.indexOf(ch)` のような MethodCall init の lowering
+3. **✅ MethodCall を含む init 式の対応** → Phase 193 完了
+   - `local digit = digits.indexOf(ch)` のような MethodCall init の lowering 完了
    - LoopBodyLocalInitLowerer 拡張（BoxCall emission）
-   - メソッド whitelist: indexOf, substring 等
+   - メソッド whitelist: indexOf, get, toString 対応
+   - 制約: body-local init のみ対応、condition 内の MethodCall は Phase 200+
 
-4. **JsonParser 残り複雑ループ・selfhost ループへの適用（Phase 194+）**
-   - `_parse_array` / `_parse_object`（P4 Continue + 複数 MethodCall）
-   - `_unescape_string`（複雑なキャリア + flatten）
-   - selfhost `.hako` コンパイラの全ループを JoinIR で処理
+4. **✅ JsonParser 実戦投入（P1/P2/P5 検証）** → Phase 194 完了
+   - 4/10 ループが JoinIR 経路で動作確認 (40% coverage)
+   - Target loops: _skip_whitespace, _trim (x2), _match_literal
+   - Deferred loops: _parse_number, _atoi (ConditionEnv constraint)
+   - Deferred loops: _parse_string, _unescape_string (complex carriers)
+   - Deferred loops: _parse_array, _parse_object (multiple MethodCalls)
+   - 詳細: phase194-loop-inventory.md, phase194-jsonparser-deployment.md
+
+5. **Pattern 3 拡張（複数キャリア対応）** → Phase 195 設計中
+   - 目的: P3（If-Else PHI）で 2-3 個の Carrier を同時処理
+   - 設計判断: ExitLine 拡張で対応（PhiGroupBox は作らない - YAGNI 原則）
+   - 対象: _parse_string（flag+buffer）、if-sum（sum+count）
+   - Phase 195-impl で実装予定
+
+6. **JsonParser 残り複雑ループへの適用（Phase 195+, 200+）**
+   - Phase 200+: ConditionEnv 拡張 (function-scoped variables) → _parse_number, _atoi
+   - Phase 195+: Pattern 3 拡張 (multi-flag carriers) → _parse_string, _unescape_string
+   - Phase 195+: MethodCall 拡張 (multiple calls in body) → _parse_array, _parse_object
+   - selfhost `.hako` コンパイラの全ループを JoinIR で処理 (Phase 210+)