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>

tools/using_e2e_smoke.sh

@@ -15,12 +15,13 @@ if [ ! -f "$APP" ]; then
   echo "[using-e2e] scaffolding sample..." >&2
   mkdir -p "$ROOT_DIR/apps/using-e2e"
   cat > "$APP" <<'NYCODE'
-// using/nyash.link E2E sample (placeholder)
+// using/nyash.link E2E sample (MVP)
+using acme.util
+
 static box Main {
   init { }
   main(args) {
-    // When using/nyash.link is active, modules can be resolved here.
-    // Placeholder just returns 0 for now.
+    // using line should be accepted when NYASH_ENABLE_USING=1
     return 0
   }
 }