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/backend/mir_interpreter/handlers/boxes_plugin.rs

@@ -55,16 +55,17 @@ pub(super) fn invoke_plugin_box(
                 &format!("{:?}", e)
             )),
         }
-    } else if let Some(string_box) = recv_box
-        .as_any()
-        .downcast_ref::<crate::boxes::string_box::StringBox>()
-    {
-        // Handle builtin StringBox methods
+    } else if recv_box.type_name() == "StringBox" {
+        // Handle builtin StringBox methods via to_string_box() so it works
+        // for both basic and plugin-backed StringBox implementations.
+        let s_box = recv_box.to_string_box();
+        let s = s_box.value;
         match method {
             "lastIndexOf" => {
                 if let Some(arg_id) = args.get(0) {
                     let needle = this.reg_load(*arg_id)?.to_string();
-                    let result_box = string_box.lastIndexOf(&needle);
+                    let helper = crate::boxes::string_box::StringBox::new(s);
+                    let result_box = helper.lastIndexOf(&needle);
                     this.write_from_box(dst, result_box);
                     Ok(())
                 } else {
@@ -74,13 +75,40 @@ pub(super) fn invoke_plugin_box(
             "indexOf" | "find" => {
                 if let Some(arg_id) = args.get(0) {
                     let needle = this.reg_load(*arg_id)?.to_string();
-                    let result_box = string_box.find(&needle);
+                    let helper = crate::boxes::string_box::StringBox::new(s);
+                    let result_box = helper.find(&needle);
                     this.write_from_box(dst, result_box);
                     Ok(())
                 } else {
                     Err(this.err_invalid("indexOf/find requires 1 argument"))
                 }
             }
+            // Phase 25.1m: minimal builtin support for StringBox.is_space(ch)
+            "is_space" => {
+                if let Some(arg_id) = args.get(0) {
+                    let ch = this.reg_load(*arg_id)?.to_string();
+                    let is_ws = ch == " " || ch == "\t" || ch == "\n" || ch == "\r";
+                    this.write_result(dst, VMValue::Bool(is_ws));
+                    Ok(())
+                } else {
+                    Err(this.err_invalid("is_space requires 1 argument"))
+                }
+            }
+            // Phase 25.1m: minimal builtin support for StringBox.is_alpha(ch)
+            "is_alpha" => {
+                if let Some(arg_id) = args.get(0) {
+                    let ch = this.reg_load(*arg_id)?.to_string();
+                    let c = ch.chars().next().unwrap_or('\0');
+                    let is_alpha =
+                        ('A'..='Z').contains(&c) ||
+                        ('a'..='z').contains(&c) ||
+                        c == '_';
+                    this.write_result(dst, VMValue::Bool(is_alpha));
+                    Ok(())
+                } else {
+                    Err(this.err_invalid("is_alpha requires 1 argument"))
+                }
+            }
             _ => Err(this.err_method_not_found("StringBox", method)),
         }
     } else {