fix(joinir): Phase 287 P2 - Pattern6 nested loop latch overwrite fix

Fix infinite loop in Pattern6 (nested loop minimal) caused by main→loop_step
overwriting k_inner_exit→loop_step latch values.

Root cause: JoinIR main entry block was incorrectly treated as BackEdge,
causing it to overwrite the correct latch incoming values set by the true
back edge (k_inner_exit → loop_step).

Solution:
- Restrict latch recording to TailCallKind::BackEdge only
- Treat only MAIN's entry block as entry-like (not loop_step's entry block)
- Add debug_assert! to detect double latch set in future

Refactoring:
- Extract latch recording to latch_incoming_recorder module (SSOT)
- Add boundary.loop_header_func_name for explicit header identification
- Strengthen tail_call_classifier with is_source_entry_like parameter

Tests: apps/tests/phase1883_nested_minimal.hako → RC:9 (was infinite loop)
Smoke: 154/154 PASS, no regressions

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

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

docs/development/current/main/10-Now.md

@@ -1,13 +1,19 @@
 # Self Current Task — Now (main)
 
-## Current Focus: Phase 188.3 (Nested Loop Lowering)
+## Current Focus: Post Phase 188.3 (Refactoring Window)
+
+**2025-12-27: Phase 188.3 完了** ✅
+- Pattern6（NestedLoopMinimal）: `apps/tests/phase1883_nested_minimal.hako` が RC=9
+- Merge SSOT（latch/entry-like/double latch）を固定（BackEdgeのみlatch記録、main entry blockのみentry-like、二重latchはdebug_assert）
+- `./tools/smokes/v2/run.sh --profile quick` 154/154 PASS 維持
+- 入口: `docs/development/current/main/phases/phase-188.3/README.md`
+- 次の指示書（refactor挟み込み）: `docs/development/current/main/phases/phase-188.3/P2-REFACTORING-INSTRUCTIONS.md`
 
 **2025-12-27: Phase 188.2 完了** ✅
 - StepTreeの `max_loop_depth` を SSOT に採用（Option A）
 - strict mode で depth > 2 を明示エラー化（Fail-Fast）
 - quick 154/154 PASS、integration selfhost FAIL=0 維持
-- 次: `docs/development/current/main/phases/phase-188.3/README.md`（depth=2 を JoinIR lowering で通す / “最小write-back”は carrier として明示）
-- 実装導線（手順書）: `docs/development/current/main/phases/phase-188.3/P1-INSTRUCTIONS.md`（merge/rewriter の “undef ValueId” 典型罠もここに固定）
+- 次: `docs/development/current/main/phases/phase-188.3/P2-REFACTORING-INSTRUCTIONS.md`（意味論不変での整理を優先）
 
 **2025-12-27: Phase S0.1 完了** ✅
 - integration selfhost を「落ちない状態」に収束（FAIL=0）