feat(loop-phi): Add body-local variable PHI generation for Rust AST loops

Phase 25.1c/k: Fix ValueId undefined errors in loops with body-local variables

**Problem:**
- FuncScannerBox.scan_all_boxes/1 and BreakFinderBox._find_loops/2 had ValueId
  undefined errors for variables declared inside loop bodies
- LoopFormBuilder only generated PHIs for preheader variables, missing body-locals
- Example: `local ch = s.substring(i, i+1)` inside loop → undefined on next iteration

**Solution:**
1. **Rust AST path** (src/mir/loop_builder.rs):
   - Detect body-local variables by comparing body_end_vars vs current_vars
   - Generate empty PHI nodes at loop header for body-local variables
   - Seal PHIs with latch + continue snapshot inputs after seal_phis()
   - Added HAKO_LOOP_PHI_TRACE=1 logging for debugging

2. **JSON v0 path** (already fixed in previous session):
   - src/runner/json_v0_bridge/lowering/loop_.rs handles body-locals
   - Uses same strategy but for JSON v0 bridge lowering

**Results:**
- ✅ FuncScannerBox.scan_all_boxes: 41 body-local PHIs generated
- ✅ Main.main (demo harness): 23 body-local PHIs generated
- ⚠️ Still some ValueId undefined errors remaining (exit PHI issue)

**Files changed:**
- src/mir/loop_builder.rs: body-local PHI generation logic
- lang/src/compiler/entry/func_scanner.hako: debug logging
- /tmp/stageb_funcscan_demo.hako: test harness

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

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

src/runner/json_v0_bridge/lowering/loop_.rs

@@ -13,6 +13,14 @@ pub(super) fn lower_loop_stmt(
     loop_stack: &mut Vec<LoopContext>,
     env: &BridgeEnv,
 ) -> Result<BasicBlockId, String> {
+    // DEBUG: Track loop lowering calls (Stage-B / JSON v0) — dev用トレース
+    if std::env::var("HAKO_LOOP_PHI_TRACE").ok().as_deref() == Some("1") {
+        eprintln!(
+            "[loop-phi/enter] lower_loop_stmt called, fn={}, base_vars={}",
+            f.signature.name,
+            vars.len()
+        );
+    }
     // Unification toggle (default ON). For now legacy path is removed; when OFF, warn and proceed unified.
     let unify_on = std::env::var("NYASH_MIR_UNIFY_LOOPFORM")
         .ok()