feat(phi): Step 5-5-D - Skip __pin$ in build_exit_phis()

Implementation:
- loopform_builder.rs:505-512: Filter __pin$ from body_local_names collection
- Result: All __pin$ variables correctly classified as BodyLocalInternal
- Exit PHI generation properly skipped for __pin$ temps

Verification (NYASH_OPTION_C_DEBUG=1):
✅ All __pin$ vars: BodyLocalInternal needs_exit_phi=false
✅ Exit PHI generation: SKIP for all __pin$ variables

Progress:
- ValueId error: 256→260 (minor variation, different root cause)
- Dominator errors: Still 1 remaining
- Test status: 267 PASS / 1 FAIL (no regressions)

Next: Investigate ValueId(260) - not a __pin$ variable

🐛 PHI Bug Option C実装: 箱分割設計で根本修正

src/mir/phi_core/loopform_builder.rs

@@ -502,6 +502,15 @@ impl LoopFormBuilder {
         let mut body_local_set: std::collections::HashSet<String> = std::collections::HashSet::new();
         for (_block_id, snapshot) in exit_snapshots {
             for var_name in snapshot.keys() {
+                // Step 5-5-D: Skip __pin$ temporary variables in exit PHI generation
+                // These are BodyLocalInternal and should NOT get exit PHIs
+                if var_name.starts_with("__pin$") && var_name.contains("$@") {
+                    if debug {
+                        eprintln!("[DEBUG/exit_phi] SKIP __pin$ variable: {}", var_name);
+                    }
+                    continue;
+                }
+
                 let is_pinned = self.pinned.iter().any(|p| &p.name == var_name);
                 let is_carrier = self.carriers.iter().any(|c| &c.name == var_name);
                 if !is_pinned && !is_carrier && !body_local_set.contains(var_name) {