feat(mir/phi): improve LoopForm parameter detection - track param names

**Problem**: is_parameter() was too simple, checking only ValueId which changes
through copies/PHIs. This caused parameters like 'data' to be misclassified as
carriers, leading to incorrect PHI construction.

**Solution**: Track original parameter names at function entry.

**Changes**:

1. **Added function_param_names field** (builder.rs):
   - HashSet<String> to track original parameter names
   - Populated in lower_static_method_as_function()
   - Cleared and repopulated for each new function

2. **Improved is_parameter()** (loop_builder.rs):
   - Check name against function_param_names instead of ValueId
   - More reliable than checking func.params (ValueIds change)
   - __pin$*$@* variables correctly classified as carriers
   - Added debug logging with NYASH_LOOPFORM_DEBUG

3. **Enhanced debug output** (loopform_builder.rs):
   - Show carrier/pinned classification during prepare_structure()
   - Show variable_map state after emit_header_phis()

**Test Results**:
- ✅ 'args' correctly identified as parameter (was working)
- ✅ 'data' now correctly identified as parameter (was broken)
- ✅ __pin variables correctly classified as carriers
- ✅ PHI values allocated and variable_map updated correctly
- ⚠️ ValueId undefined errors persist (separate issue)

**Remaining Issue**:
ValueId(10) undefined error suggests PHI visibility problem or VM verification
issue. Needs further investigation of emit_phi_at_block_start() or VM executor.

**Backward Compatibility**:
- Flag OFF: 100% existing behavior preserved (legacy path unchanged)
- Feature-flagged with NYASH_LOOPFORM_PHI_V2=1

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/mir/builder.rs

@@ -109,6 +109,10 @@ pub struct MirBuilder {
     /// Index of static methods seen during lowering: name -> [(BoxName, arity)]
     pub(super) static_method_index: std::collections::HashMap<String, Vec<(String, usize)>>,
 
+    /// Function parameter names (for LoopForm PHI construction)
+    /// Tracks the original parameter names at function entry
+    pub(super) function_param_names: HashSet<String>,
+
     /// Fast lookup: method+arity tail → candidate function names (e.g., ".str/0" → ["JsonNode.str/0", ...])
     pub(super) method_tail_index: std::collections::HashMap<String, Vec<String>>,
     /// Source size snapshot to detect when to rebuild the tail index
@@ -191,9 +195,10 @@ impl MirBuilder {
             plugin_method_sigs,
             current_static_box: None,
             static_method_index: std::collections::HashMap::new(),
+            function_param_names: HashSet::new(),
             method_tail_index: std::collections::HashMap::new(),
             method_tail_index_source_len: 0,
-            
+
             loop_header_stack: Vec::new(),
             loop_exit_stack: Vec::new(),
             if_merge_stack: Vec::new(),
@@ -549,6 +554,34 @@ impl MirBuilder {
         }
     }
 
+    /// Update an existing PHI instruction's inputs (for loop sealing)
+    /// Used by LoopFormBuilder to complete incomplete PHI nodes
+    pub(super) fn update_phi_instruction(
+        &mut self,
+        block: BasicBlockId,
+        phi_id: ValueId,
+        new_inputs: Vec<(BasicBlockId, ValueId)>,
+    ) -> Result<(), String> {
+        if let Some(ref mut function) = self.current_function {
+            if let Some(block_data) = function.get_block_mut(block) {
+                // Find PHI instruction with matching dst
+                for inst in &mut block_data.instructions {
+                    if let MirInstruction::Phi { dst, inputs } = inst {
+                        if *dst == phi_id {
+                            *inputs = new_inputs;
+                            return Ok(());
+                        }
+                    }
+                }
+                Err(format!("PHI instruction {} not found in block {}", phi_id, block))
+            } else {
+                Err(format!("Block {} not found", block))
+            }
+        } else {
+            Err("No current function".to_string())
+        }
+    }
+
     // フェーズM: is_no_phi_mode()メソッド削除
 
     // フェーズM: insert_edge_copy()メソッド削除（no_phi_mode撤廃により不要）