refactor(joinir): unify boundary join_inputs SSOT (pattern4/6/7)

Apply Phase 256.8 SSOT fix to Pattern4/6/7:
- Use join_module.entry.params.clone() instead of hardcoded ValueIds
- Add fail-fast validation for params count mismatch
- Remove ValueId(0), ValueId(PARAM_MIN + k) patterns
- Clean up unused PARAM_MIN imports

This prevents entry_param_mismatch errors structurally and maintains
consistency with Pattern2/3.

Changes:
- pattern4_with_continue.rs: Lines 442-476 (SSOT extraction + validation)
- pattern6_scan_with_init.rs: Lines 447-471 (SSOT extraction + validation)
- pattern7_split_scan.rs: Lines 495-526 (SSOT extraction + validation)

All patterns now use the same SSOT principle:
1. Extract entry function (priority: join_module.entry → fallback "main")
2. Use params as SSOT: join_inputs = entry_func.params.clone()
3. Build host_inputs in expected order (pattern-specific)
4. Fail-fast validation: join_inputs.len() == host_inputs.len()

Verification:
- cargo build --release: ✅ PASS (no PARAM_MIN warnings)
- Quick profile: ✅ First FAIL still json_lint_vm (baseline maintained)
- Pattern6 smoke: ✅ PASS (index_of test)
- Pattern7 smoke: Pre-existing phi pred mismatch (not introduced by SSOT)

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

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

docs/development/current/main/10-Now.md

@@ -45,12 +45,14 @@
   - Pattern6/index_of が VM/LLVM で PASS
   - `loop_invariants` を導入して ConditionOnly 誤用を根治
 
-## 2025-12-19：Phase 256（StringUtils.split/2 可変 step ループ）🔜
+## 2025-12-19：Phase 256（StringUtils.split/2 可変 step ループ）✅
 
 - Phase 256 README: `docs/development/current/main/phases/phase-256/README.md`
-- Current first FAIL: `json_lint_vm`（Pattern2 break cond: `this.is_whitespace(...)` needs `current_static_box_name`）
-- 状況:
-  - `MirInstruction::Select` の導入は完了、Pattern6（index_of）は PASS 維持。
+- Status:
+  - `StringUtils.split/2` は VM `--verify` / integration smoke まで PASS
+  - `--profile quick` の最初の FAIL は Phase 257（`StringUtils.last_index_of/2`）へ移動
+  - 設計SSOT: `docs/development/current/main/design/join-explicit-cfg-construction.md`
+- 直近の主要fix:
   - `ValueId(57)` undefined は根治（原因は `const_1` 未初期化）。
   - SSA undef（`%49/%67`）は P1.7 で根治（continuation 関数名の SSOT 不一致）。
   - P1.8で ExitLine/jump_args の余剰許容と関数名マッピングを整流。
@@ -59,6 +61,13 @@
   - P1.11で ExitArgsCollector の expr_result slot 判定を明確化し、split が `--verify` / integration smoke まで PASS。
   - P1.5-DBG: boundary entry params の契約チェックを追加（VM実行前 fail-fast）。
   - P1.6: 契約チェックの薄い集約 `run_all_pipeline_checks()` を導入（pipeline の責務を縮退）。
+  - P1.13: Pattern2 boundary entry params を `join_module.entry.params` SSOT へ寄せた（ValueId 推測生成の撤去）。
+
+## 2025-12-20：Phase 257（last_index_of early return loop）🔜
+
+- Phase 257 README: `docs/development/current/main/phases/phase-257/README.md`
+- Goal: `StringUtils.last_index_of/2` を JoinIR で受理し、`--profile quick` を緑に戻す
+  - Investigation（最小再現/論点）: `docs/development/current/main/investigations/phase-257-last-index-of-loop-shape.md`
 
 ## 2025-12-19：Phase 254（index_of loop pattern）✅ 完了（Blocked by Phase 255）