refactor(joinir): Phase 286 P3.2 cleanup + normalizer modularization

## Legacy Pattern5 削除 (488行)
- pattern5_infinite_early_exit.rs 完全削除
- LOOP_PATTERNS テーブルからエントリ削除
- Plan extractor が SSOT

## Normalizer 分割 (3294行 → 12ファイル)
- helpers.rs: 共通ヘルパー関数
- pattern*.rs: 各パターン専用ファイル
- mod.rs: ディスパッチャ

## ドキュメント更新
- Phase 286 README: クリーンアップ完了・Fail-Fast方針記載
- Phase 287 README: 将来計画

quick smoke 154/154 PASS

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

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

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

@@ -79,7 +79,6 @@ pub(in crate::mir::builder) mod pattern2_with_break;
 pub(in crate::mir::builder) mod pattern3_with_if_phi;
 pub(in crate::mir::builder) mod pattern4_carrier_analyzer;
 pub(in crate::mir::builder) mod pattern4_with_continue;
-pub(in crate::mir::builder) mod pattern5_infinite_early_exit; // Phase 131-11
 pub(in crate::mir::builder) mod pattern6_scan_with_init; // Phase 254 P0: index_of/find/contains pattern
 pub(in crate::mir::builder) mod pattern7_split_scan; // Phase 256 P0: split/tokenization with variable step
 pub(in crate::mir::builder) mod pattern8_scan_bool_predicate; // Phase 259 P0: boolean predicate scan (is_integer/is_valid)