feat(joinir): Phase 259 P0 complete - Pattern8 final fixes + docs (pre-block-params migration)

Phase 259 P0: Pattern8 (BoolPredicateScan) 完全完了
is_integer/1 を Pattern8 で受理し、VM/LLVM EXE 両方で動作確認完了。
次の大工事（block-parameterized CFG への移行）前のマイルストーンとして記録。

## Key Fixes Applied

1. **skipped_entry_redirects** (instruction_rewriter.rs)
   - k_exit のスキップ時、entry block 参照を exit_block_id へリダイレクト
   - BasicBlockId not found エラーを根治

2. **loop_var_name** (pattern8_scan_bool_predicate.rs)
   - merge_entry_block 選択に使用（`Some(parts.loop_var.clone())`）
   - 未設定時の誤った entry block 選択を修正

3. **loop_invariants** (pattern8_scan_bool_predicate.rs)
   - PHI-free 不変量パラメータ（`[(me, me_host), (s, s_host)]`）
   - loop_var_name 設定時、BoundaryInjector が join_inputs Copy を全スキップするため必要
   - Pattern6 と同じ設計（header PHI で不変量を保持）

4. **expr_result** (pattern8_scan_bool_predicate.rs)
   - k_exit からの返り値を明示設定（`Some(join_exit_value)`）
   - Pattern7 style（推測ではなく明示）

5. **Smoke test scripts**
   - set +e パターンで exit code 7 をキャプチャ
   - LLVM EXE スクリプトにコメント追加（tools/build_llvm.sh 経由の明記）

## Contract Documentation

- join-explicit-cfg-construction.md に Pattern8 契約の具体例を追加
  - "pattern増でも推測増にしない" の実例として記録
  - loop_var_name / loop_invariants / expr_result / jump_args_layout の契約を明示
- 20-Decisions.md に正規化（Semantic/Plumbing）の分離方針を追記
- DOCS_LAYOUT.md に重要ドキュメントへの参照を追加

## Test Results

- ✅ VM smoke test: `[PASS] phase259_p0_is_integer_vm` (exit 7)
- ✅ LLVM EXE: tools/build_llvm.sh 経由で exit 7 確認
- ✅ --verify: PASS

## Next FAIL (Phase 260+)

- Function: `Main.main/0` in `apps/examples/json_lint/main.hako`
- Error: `[cf_loop/pattern2] Failed to extract break condition from loop body`
- Pattern: Nested loop（外側 loop + 内側 loop with break）

🚀 次の大工事: block-parameterized CFG への移行を開始します。

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

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/mir/builder/control_flow/joinir/patterns/pattern8_scan_bool_predicate.rs

@@ -506,13 +506,34 @@ impl MirBuilder {
             promoted_bindings: std::collections::BTreeMap::new(),
         };
 
+        // Phase 259 P0 FIX: Create loop_invariants for me and s
+        // These are passed to the loop but don't change across iterations.
+        // Order MUST match JoinModule loop_step params: [i, me, s]
+        // carrier_order is built as: [loop_var] + loop_invariants
+        let loop_invariants = vec![
+            ("me".to_string(), me_host),     // me (receiver) → JoinIR param 1
+            (parts.haystack.clone(), s_host), // s (haystack) → JoinIR param 2
+        ];
+
+        if debug {
+            trace.debug(
+                "pattern8/lower",
+                &format!(
+                    "Phase 259 P0: CarrierInfo with loop_var only (i: LoopState), {} loop_invariants (me, s)",
+                    loop_invariants.len()
+                ),
+            );
+        }
+
         // Phase 259 P0: expr_result = join_exit_value (Pattern7 style)
         // Pattern8 returns boolean from k_exit, not loop variable
 
         let boundary = JoinInlineBoundaryBuilder::new()
             .with_inputs(join_inputs, host_inputs)
+            .with_loop_invariants(loop_invariants) // Phase 259 P0 FIX: Add loop invariants for me and s
             .with_exit_bindings(exit_bindings)
             .with_carrier_info(carrier_info)
+            .with_loop_var_name(Some(parts.loop_var.clone())) // Phase 259 P0 FIX: Required for merge entry selection
             .with_expr_result(Some(join_exit_value)) // ✅ CRITICAL: Set expr_result to k_exit param
             .build();