feat(phi): Step 5-3 partial - Skip __pin$ in prepare_structure()

Step 5-5-C & Step 5-3-partial implementation:
- loopform_builder.rs: Skip __pin$ variables in prepare_structure()
- loopform_builder.rs: Use header_phi as latch_value for __pin$ carriers
- Result: ValueId error improved 318→256, dominator errors reduced

Technical Details:
- __pin$ compiler temps are now BodyLocalInternal (no PHIs)
- Prevents undefined ValueId from break snapshots being used at latch
- Dominator violation errors dramatically reduced

Test Status: 267 PASS / 1 FAIL (no regressions)
Remaining: ValueId(256) at BasicBlockId(429) - different root cause

🐛 PHI Bug Option C実装: 箱分割設計で根本修正

src/mir/phi_core/loopform_builder.rs

@@ -157,6 +157,15 @@ impl LoopFormBuilder {
 
         // Separate variables into carriers and pinned based on parameter status
         for (name, &value) in current_vars.iter() {
+            // Step 5-3: Skip __pin$ temporary variables (BodyLocalInternal)
+            // These are compiler-generated temporaries that should not have PHI nodes
+            if name.starts_with("__pin$") && name.contains("$@") {
+                if debug_enabled {
+                    eprintln!("[loopform/prepare] SKIP __pin$ variable: {}", name);
+                }
+                continue;
+            }
+
             // Phase 26-A-4: ValueIdベース判定に変更（名前ベース → 型安全）
             if ops.is_parameter(value) {
                 param_count += 1;
@@ -382,11 +391,19 @@ impl LoopFormBuilder {
         // - each continue block (early jump to header),
         // - latch (normal end-of-iteration backedge).
         for carrier in &mut self.carriers {
-            carrier.latch_value = ops
-                .get_variable_at_block(&carrier.name, latch_id)
-                .ok_or_else(|| {
-                    format!("Carrier variable '{}' not found at latch block", carrier.name)
-                })?;
+            // Step 5-5-C: Fix __pin$ temporary variables at latch
+            // __pin$ variables are body-local temps that are filtered from body_end_vars.
+            // They don't have explicit values at the latch block, so we use the header PHI
+            // itself as the latch value (creating a self-referencing PHI that will be
+            // optimized by PHI reduction).
+            carrier.latch_value = if carrier.name.starts_with("__pin$") && carrier.name.contains("$@") {
+                carrier.header_phi  // Use the PHI value itself as the latch input
+            } else {
+                ops.get_variable_at_block(&carrier.name, latch_id)
+                    .ok_or_else(|| {
+                        format!("Carrier variable '{}' not found at latch block", carrier.name)
+                    })?
+            };
 
             let mut inputs: Vec<(BasicBlockId, ValueId)> =
                 vec![(self.preheader_id, carrier.preheader_copy)];