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>

lang/src/compiler/entry/func_scanner.hako

@@ -4,7 +4,7 @@
 // Scope: method <name>(params) { ... } outside of main (same box Main)
 // Output: [{"name":"<name>","params":[...],"body_json":"<Program JSON>","box":"Main"}]
 
-using lang.compiler.parser.box as ParserBox
+using "lang.compiler.parser.box" as ParserBox
 
 static box FuncScannerBox {
   // Scan source for method definitions (excluding Main.main)
@@ -19,6 +19,8 @@ static box FuncScannerBox {
 	    local defs = new ArrayBox()
 	    local s = "" + source
     local n = s.length()
+    // DEBUG: 一時的に常時トレースを有効化（Stage‑B fib 用の挙動観測）
+    print("[funcscan/scan_all_boxes] source_len=" + ("" + n))
     local i = 0
     local in_str = 0
     local esc = 0
@@ -55,6 +57,13 @@ static box FuncScannerBox {
         if ch2 == "*" { in_block = 1  i = i + 2  continue }
       }
 
+      // DEBUG: "box" キーワード候補をチェック（Stage‑B fib 用）
+      if i + 3 <= n {
+        local candidate = s.substring(i, i + 3)
+        if candidate == "box" {
+          print("[funcscan/box_candidate] i=" + ("" + i) + " candidate=" + candidate)
+        }
+      }
       if s.substring(i, i + 3) == "box" && me._kw_boundary_before(s, i) == 1 && me._kw_boundary_after(s, i + 3) == 1 {
         local cursor = i + 3
         cursor = me._skip_whitespace(s, cursor)