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/phi_core/loopform_builder.rs

@@ -38,7 +38,7 @@ pub struct PinnedVariable {
 ///
 /// Key Innovation: All ValueIds allocated upfront before any MIR emission,
 /// eliminating circular dependency issues.
-#[derive(Debug, Default)]
+#[derive(Debug)]
 pub struct LoopFormBuilder {
     pub carriers: Vec<CarrierVariable>,
     pub pinned: Vec<PinnedVariable>,
@@ -77,6 +77,10 @@ impl LoopFormBuilder {
                     preheader_copy: ops.new_value(), // Allocate NOW
                     header_phi: ops.new_value(),     // Allocate NOW
                 };
+                if std::env::var("NYASH_LOOPFORM_DEBUG").is_ok() {
+                    eprintln!("[loopform/prepare] pinned: {} -> init={:?}, copy={:?}, phi={:?}",
+                             name, value, pinned.preheader_copy, pinned.header_phi);
+                }
                 self.pinned.push(pinned);
             } else {
                 // Carrier variable (local, modified in loop)
@@ -85,8 +89,12 @@ impl LoopFormBuilder {
                     init_value: value,
                     preheader_copy: ops.new_value(), // Allocate NOW
                     header_phi: ops.new_value(),     // Allocate NOW
-                    latch_value: ValueId::INVALID,   // Will be set during seal
+                    latch_value: ValueId(0),         // Will be set during seal (placeholder)
                 };
+                if std::env::var("NYASH_LOOPFORM_DEBUG").is_ok() {
+                    eprintln!("[loopform/prepare] carrier: {} -> init={:?}, copy={:?}, phi={:?}",
+                             name, value, carrier.preheader_copy, carrier.header_phi);
+                }
                 self.carriers.push(carrier);
             }
         }