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>

dev/selfhosting/README.md

@@ -17,6 +17,10 @@ Quickstart
 - Bootstrap c0→c1→c1' (optional):
   - `./tools/bootstrap_selfhost_smoke.sh`
 
+Docs
+
+- One‑page guide: `docs/self-hosting.md`
+
 Flags
 
 - `NYASH_DISABLE_PLUGINS=1`: stabilize core path
@@ -28,4 +32,3 @@ Tips
 
 - For debug, set `NYASH_CLI_VERBOSE=1`.
 - Keep temp artifacts under this folder (`dev/selfhosting/_tmp/`) to avoid polluting repo root.
-