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/tests/mir_pure_e2e_vm.rs

@@ -1,7 +1,8 @@
 #[cfg(test)]
 mod tests {
-    use crate::parser::NyashParser;
     use crate::backend::VM;
+    use crate::parser::NyashParser;
+    use crate::mir::{MirCompiler, MirPrinter, MirVerifier};
 
     #[test]
     fn vm_exec_new_string_length_under_pure_mode() {
@@ -26,5 +27,38 @@ return (new StringBox("Hello")).length()
         // Cleanup
         std::env::remove_var("NYASH_MIR_CORE13_PURE");
     }
-}
 
+    /// Minimal smoke for MirInstruction::DebugLog
+    #[test]
+    fn mir_debuglog_minimal_printer_and_verifier() {
+        std::env::set_var("NYASH_PARSER_STAGE3", "1");
+        std::env::set_var("NYASH_PARSER_ALLOW_SEMICOLON", "1");
+        std::env::set_var("NYASH_MIR_DEBUG_LOG", "1");
+
+        let src = r#"
+static box Main {
+  method main(args) {
+    local x = 1
+    local y = 2
+    // DebugLog 自体の存在が Verifier/Printer で問題にならないことの確認用。
+    return x + y
+  }
+}
+"#;
+
+        let ast = NyashParser::parse_from_string(src).expect("parse ok");
+        let mut mc = MirCompiler::with_options(false);
+        let cr = mc.compile(ast).expect("compile ok");
+
+        let mut verifier = MirVerifier::new();
+        if let Err(errors) = verifier.verify_module(&cr.module) {
+            for e in &errors {
+                eprintln!("[mir-debuglog] {}", e);
+            }
+            panic!("MIR verification failed for DebugLog minimal case");
+        }
+
+        let dump = MirPrinter::new().print_module(&cr.module);
+        eprintln!("----- MIR DUMP (DebugLog.min) -----\n{}", dump);
+    }
+}