docs: update Phase 257-259 SSOT (first FAIL is is_integer)

docs/development/current/main/phases/phase-258/README.md

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+Status: Completed (2025-12-20)
+Scope: `StringUtils.index_of_string/2`（dynamic window scan）を JoinIR で受理して `--profile quick` を進める。
+Related:
+- Now: `docs/development/current/main/10-Now.md`
+- Phase 257: `docs/development/current/main/phases/phase-257/README.md`
+- Phase 259 (Next): `docs/development/current/main/phases/phase-259/README.md`
+- Design goal: `docs/development/current/main/design/join-explicit-cfg-construction.md`
+
+# Phase 258: `StringUtils.index_of_string/2` (dynamic window scan)
+
+## Current Status (SSOT)
+
+- ✅ **P0 完了**（2025-12-21）
+- Former first FAIL: `json_lint_vm / StringUtils.index_of_string/2`
+- **New first FAIL**: `json_lint_vm / StringUtils.is_integer/1`（Phase 259へ移行）
+- Shape summary (from quick log):
+  - prelude: `if substr.length()==0 return 0`, `if substr.length()>s.length() return -1`
+  - loop: `loop(i <= s.length() - substr.length()) { if s.substring(i, i+substr.length()) == substr return i; i=i+1 }`
+  - post: `return -1`
+  - caps: `If,Loop,Return`
+
+## Goal
+
+- ✅ `./tools/smokes/v2/run.sh --profile quick` が `StringUtils.index_of_string/2` を突破し、次の FAIL へ進む
+
+## Proposed Approach (P0)
+
+方針: Pattern6（ScanWithInit）を “needle length 付き scan” に最小拡張する（構造で解決）
+
+Pattern6 との差分だけを足す:
+- loop cond: `i <= s.length() - substr.length()`（bound 付き）
+- window: `substring(i, i + substr.length())`
+- needle: `substr`（String）
+
+JoinIR 側は P0 では “毎回 length() を呼んでよい”。まず correctness を固定:
+- `needle_len = substr.length()`
+- `bound = s.length() - needle_len`
+- stop: `i > bound` で k_exit（not-found return）
+- match: `s.substring(i, i + needle_len) == substr` で return i
+- step: `i = i + 1`
+
+## Tasks (Draft)
+
+1) Fixture + integration smokes
+2) Pattern6 extractor を拡張して `index_of_string/2` の loop 形を accept（Fail-Fast）
+3) Pattern6 lowerer を拡張（dynamic window length / bound）して JoinIR を生成
+4) `--verify` / integration / quick を回して SSOT 更新