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/mir/instruction/methods.rs

@@ -48,7 +48,9 @@ impl MirInstruction {
             MirInstruction::NewBox { .. } => EffectMask::PURE.add(Effect::Alloc),
 
             // Debug has debug effect
-            MirInstruction::Debug { .. } => EffectMask::PURE.add(Effect::Debug),
+            MirInstruction::Debug { .. } | MirInstruction::DebugLog { .. } => {
+                EffectMask::PURE.add(Effect::Debug)
+            }
 
             // Print has external write effect
             MirInstruction::Print { effects, .. } => *effects,
@@ -126,6 +128,7 @@ impl MirInstruction {
             | MirInstruction::Return { .. }
             | MirInstruction::ArraySet { .. }
             | MirInstruction::Debug { .. }
+            | MirInstruction::DebugLog { .. }
             | MirInstruction::Print { .. }
             | MirInstruction::Throw { .. }
             | MirInstruction::RefSet { .. }
@@ -176,6 +179,7 @@ impl MirInstruction {
                 index,
                 value,
             } => vec![*array, *index, *value],
+            MirInstruction::DebugLog { values, .. } => values.clone(),
 
             MirInstruction::Branch { condition, .. } => vec![*condition],
 
@@ -263,4 +267,4 @@ impl ConstValue {
         }
     }
     */
-}
+}