refactor(joinir): Phase 287 P2 - Strengthen BackEdge/latch conditions (WIP)

**Problem**: Phase 188.3 Pattern6 (nested loop) encounters infinite loop
- inner_step → inner_step (self-recursion) incorrectly classified as BackEdge
- → redirects to outer header (loop_step) instead of inner_step entry
- is_recursive_call causes inner recursion to overwrite outer latch values
- → PHI receives wrong values → i doesn't increment → infinite loop

**Fix 1: BackEdge classification strictness** (tail_call_classifier.rs)
- Add `is_target_loop_entry` parameter to classify_tail_call()
- BackEdge ONLY when target==loop_step (entry_func), not inner_step
- Prevents inner_step → inner_step from redirecting to outer header

**Fix 2: latch_incoming guard** (instruction_rewriter.rs)
- Change condition from `is_recursive_call || is_target_loop_entry`
  to `is_target_loop_entry` only
- Prevents inner_step self-recursion from overwriting outer loop's latch
- set_latch_incoming() now called with correct values (verified by debug)

**Status**: 🚧 WIP - Infinite loop still occurs
- set_latch_incoming('i', BasicBlockId(8), ValueId(21)) ✅ Called correctly
- But final PHI: `phi [%4, bb8]` instead of `phi [%21, bb8]` ❌
- Root cause likely in PHI generation (merge/mod.rs), not latch_incoming
- Next: Investigate why latch_incoming values aren't used in PHI

**Files**:
- src/mir/builder/control_flow/joinir/merge/tail_call_classifier.rs
- src/mir/builder/control_flow/joinir/merge/instruction_rewriter.rs

🤖 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

@@ -6,7 +6,8 @@
 - 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 / Phase 188.3は“最小のwrite-back”をcarrierとして明示、一般化は次フェーズ）
+- 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）
 
 **2025-12-27: Phase S0.1 完了** ✅
 - integration selfhost を「落ちない状態」に収束（FAIL=0）