Merge selfhosting-dev into main (Core-13 pure CI/tests + LLVM bridge) (#126)

* WIP: sync before merging origin/main

* fix: unify using/module + build CLI; add missing helper in runner; build passes; core smokes green; jit any.len string now returns 3

* Apply local changes after merging main; keep docs/phase-15 removed per main; add phase-15.1 docs and tests

* Remove legacy docs/phase-15/README.md to align with main

* integration: add Core-13 pure CI, tests, and minimal LLVM execute bridge (no docs) (#125)

Co-authored-by: Tomoaki <tomoaki@example.com>

---------

Co-authored-by: Selfhosting Dev <selfhost@example.invalid>
Co-authored-by: Tomoaki <tomoaki@example.com>

.github/pull_request_template.md

@@ -9,3 +9,8 @@
 - [ ] 回帰CI green（env直読み検出なし）
 - [ ] stats: fallback率・理由が記録される
 
+### Selfhosting‑dev Gate（このブランチ向け）
+- [ ] `bash tools/selfhost_vm_smoke.sh` が PASS（plugins 無効）
+- [ ] `docs/CONTRIBUTING-MERGE.md` の境界方針を満たす（Cranelift実装差分は専用ブランチ）
+- 影響範囲: runner / interpreter / vm / tools / docs
+- Feature gates（該当時）: `cranelift-jit`, その他（記述）