feat(phi): Step 5-5-F/G - Prevent __pin$ from entering variable_map

Step 5-5-F: build_assignment() safeguard
- Skip inserting __pin$ temporaries into variable_map
- __pin$ variables are transient compiler-generated temps
- They should not persist across blocks or loops

Step 5-5-G: build_variable_access() safeguard
- Reject attempts to access __pin$ variables from variable_map
- Return clear error: "COMPILER BUG: Attempt to access __pin$ temporary"
- This catches stale __pin$ references early

Root Cause (Corrected):
- NOT BlockId renumbering (as Task initially thought)
- ChatGPT analysis: variable_map/snapshot/carrier have wrong ValueIds
- __pin$ temps were persisting and causing stale references

Test Status: 267 PASS / 1 FAIL (no regressions)

Next: Detailed MIR dump + VM trace analysis to find exact ValueId source

🐛 PHI Bug Option C実装: 箱分割設計で根本修正

src/mir/builder.rs

@@ -456,6 +456,17 @@ impl MirBuilder {
 
     /// Build variable access
     pub(super) fn build_variable_access(&mut self, name: String) -> Result<ValueId, String> {
+        // Step 5-5-G: __pin$ variables should NEVER be accessed from variable_map
+        // They are transient temporaries created during expression building and
+        // should not persist across blocks. If we see one here, it's a compiler bug.
+        if name.starts_with("__pin$") {
+            return Err(format!(
+                "COMPILER BUG: Attempt to access __pin$ temporary '{}' from variable_map. \
+                 __pin$ variables should only exist as direct SSA values, not as named variables.",
+                name
+            ));
+        }
+
         if let Some(&value_id) = self.variable_map.get(&name) {
             Ok(value_id)
         } else {
@@ -500,8 +511,18 @@ impl MirBuilder {
         // SSA + PHI merges work correctly without explicit pinning here.
         // The expression building already creates necessary temporaries.
 
-        // In SSA form, each assignment creates a new value
-        self.variable_map.insert(var_name.clone(), value_id);
+        // Step 5-5-F: NEVER insert __pin$ temporaries into variable_map
+        // __pin$ variables are transient compiler-generated temporaries that should
+        // never be tracked as real variables. They are used only within expression
+        // building and should not persist across blocks or loops.
+        //
+        // BUG FIX: Previously, __pin$ variables would be inserted into variable_map,
+        // causing stale references after LoopForm transformation renumbers blocks.
+        // Result: VM would try to read undefined ValueIds (e.g., ValueId(270) at bb303).
+        if !var_name.starts_with("__pin$") {
+            // In SSA form, each assignment creates a new value
+            self.variable_map.insert(var_name.clone(), value_id);
+        }
 
         Ok(value_id)
     }