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/shared/common/string_helpers.hako

@@ -97,6 +97,14 @@ static box StringHelpers {
   starts_with(src, i, pat) {
     local n = src.length()
     local m = pat.length()
+    {
+      // Dev-only trace for Stage‑B / parser調査用
+      local dbg = env.get("HAKO_STAGEB_DEBUG")
+      if dbg != null && ("" + dbg) == "1" {
+        print("[string_helpers/starts_with] src=\"" + src + "\"")
+        print("[string_helpers/starts_with] i=" + ("" + i) + " m=" + ("" + m) + " n=" + ("" + n))
+      }
+    }
     if i + m > n { return 0 }
     local k = 0
     loop(k < m) {
@@ -108,7 +116,15 @@ static box StringHelpers {
 
   // Keyword match with word boundary (next char not [A-Za-z0-9_])
   starts_with_kw(src, i, kw) {
-    if me.starts_with(src, i, kw) == 0 { return 0 }
+    {
+      local dbg = env.get("HAKO_STAGEB_DEBUG")
+      if dbg != null && ("" + dbg) == "1" {
+        print("[string_helpers/starts_with_kw] src=\"" + src + "\" i=" + ("" + i) + " kw=\"" + kw + "\"")
+      }
+    }
+    // 同一箱内ヘルパーとして、receiver 経由ではなく
+    // 明示的な箱名付きの静的呼び出しにする（引数順のずれ防止）。
+    if StringHelpers.starts_with(src, i, kw) == 0 { return 0 }
     local n = src.length()
     local j = i + kw.length()
     if j >= n { return 1 }