fix(mir/exit_phi): Pass branch_source_block to build_exit_phis()

## Problem
Exit PHI generation was using header_id as predecessor, but when
build_expression(condition) creates new blocks, the actual branch
instruction is emitted from a different block, causing:
"phi pred mismatch: no input for predecessor BasicBlockId(X)"

## Solution
- Modified build_exit_phis() to accept branch_source_block parameter
- Capture actual block after condition evaluation in loop_builder.rs
- Use branch_source_block instead of header_id for PHI inputs

## Progress
- Error changed from ValueId(5941)/BasicBlockId(4674) to
  ValueId(5927)/BasicBlockId(4672), showing partial fix
- Added comprehensive test suite in mir_loopform_exit_phi.rs
- Added debug logging to trace condition block creation

## Status
Partial fix - unit tests pass, but Test 2 (Stage-B compilation) still
has errors. Needs further investigation of complex nested compilation
scenarios.

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

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

src/mir/loop_builder.rs

@@ -105,6 +105,8 @@ impl<'a> LoopBuilder<'a> {
         // Snapshot variables at break point for exit PHI generation
         let snapshot = self.get_current_variable_map();
         let cur_block = self.current_block()?;
+        eprintln!("[DEBUG/do_break] Saved snapshot from block {:?}, vars: {:?}",
+            cur_block, snapshot.keys().collect::<Vec<_>>());
         self.exit_snapshots.push((cur_block, snapshot));
 
         if let Some(exit_bb) = crate::mir::builder::loops::current_exit(self.parent_builder) {
@@ -237,8 +239,20 @@ impl<'a> LoopBuilder<'a> {
         self.exit_snapshots.clear();
 
         // Emit condition check in header
+        if std::env::var("NYASH_LOOPFORM_DEBUG").is_ok() {
+            eprintln!("[loopform/condition] BEFORE build_expression: current_block={:?}", self.current_block()?);
+        }
         let cond_value = self.parent_builder.build_expression(condition)?;
+        // Capture the ACTUAL block that emits the branch (might differ from header_id
+        // if build_expression created new blocks)
+        let branch_source_block = self.current_block()?;
+        if std::env::var("NYASH_LOOPFORM_DEBUG").is_ok() {
+            eprintln!("[loopform/condition] AFTER build_expression: branch_source_block={:?}", branch_source_block);
+        }
         self.emit_branch(cond_value, body_id, exit_id)?;
+        if std::env::var("NYASH_LOOPFORM_DEBUG").is_ok() {
+            eprintln!("[loopform/condition] AFTER emit_branch: current_block={:?}", self.current_block()?);
+        }
 
         // Lower loop body
         self.set_current_block(body_id)?;
@@ -280,7 +294,7 @@ impl<'a> LoopBuilder<'a> {
 
         // Build exit PHIs for break statements
         let exit_snaps = self.exit_snapshots.clone();
-        loopform.build_exit_phis(self, exit_id, &exit_snaps)?;
+        loopform.build_exit_phis(self, exit_id, branch_source_block, &exit_snaps)?;
 
         // Pop loop context
         crate::mir::builder::loops::pop_loop_context(self.parent_builder);
@@ -341,7 +355,7 @@ impl<'a> LoopBuilder<'a> {
         // 4. ループ変数のPhi nodeを準備
         // ここでは、ループ内で変更される可能性のある変数を事前に検出するか、
         // または変数アクセス時に遅延生成する（再束縛は条件式構築後に行う）
-        let incs = self.prepare_loop_variables(header_id, preheader_id, &pre_vars_snapshot)?;
+        let incs = self.prepare_loop_variables(header_id, preheader_id, &pre_vars_snapshot, &assigned_vars)?;
 
         // 5. 条件評価（Phi nodeの結果を使用）
         // Heuristic pre-pin: if condition is a comparison, evaluate its operands and pin them
@@ -566,18 +580,36 @@ impl<'a> LoopBuilder<'a> {
     // PHI Helpers — prepare/finalize PHIs and block sealing
     // =============================================================
     /// ループ変数の準備（事前検出または遅延生成）
+    ///
+    /// ポリシー:
+    /// - ループキャリア（ループ本体で再代入される変数）と pinned 変数のみを PHI 対象とする。
+    /// - ループ不変のローカル（text_len / pattern_len など）は preheader 値をそのまま使い、
+    ///   不要な PHI を張らないことで SSA 破綻（同一 ValueId の二重定義）を防ぐ。
     fn prepare_loop_variables(
         &mut self,
         header_id: BasicBlockId,
         preheader_id: BasicBlockId,
         pre_vars_snapshot: &std::collections::HashMap<String, ValueId>,
+        assigned_vars: &[String],
     ) -> Result<Vec<crate::mir::phi_core::loop_phi::IncompletePhi>, String> {
         use std::sync::atomic::{AtomicUsize, Ordering};
         static CALL_COUNT: AtomicUsize = AtomicUsize::new(0);
         let count = CALL_COUNT.fetch_add(1, Ordering::SeqCst);
 
-        // Use the variable map captured at preheader (before switching to header)
-        let current_vars = pre_vars_snapshot.clone();
+        // Use the variable map captured at preheader (before switching to header),
+        // but filter to:
+        // - ループキャリア（assigned_vars に含まれる変数）
+        // - pinned 変数（__pin$*）: 受信箱など、ループをまたいで値を運ぶ必要があるもの
+        let mut current_vars = std::collections::HashMap::new();
+        for (name, &val) in pre_vars_snapshot.iter() {
+            if name.starts_with("__pin$") {
+                current_vars.insert(name.clone(), val);
+                continue;
+            }
+            if assigned_vars.iter().any(|v| v == name) {
+                current_vars.insert(name.clone(), val);
+            }
+        }
         // Debug: print current_vars before prepare (guarded by env)
         let dbg = std::env::var("NYASH_BUILDER_DEBUG").ok().as_deref() == Some("1");
         if dbg {