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_strict_path_fail_smoke.sh

@@ -0,0 +1,32 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+SCRIPT_DIR=$(CDPATH= cd -- "$(dirname -- "$0")" && pwd)
+ROOT_DIR=$(CDPATH= cd -- "$SCRIPT_DIR/.." && pwd)
+BIN="$ROOT_DIR/target/release/nyash"
+
+if [ ! -x "$BIN" ]; then
+  cargo build --release --features cranelift-jit >/dev/null
+fi
+
+TMP=$(mktemp)
+cat >"$TMP" <<'NY'
+using "./definitely_missing_12345.nyash" as Miss
+static box Main { main(args) { return 0 } }
+NY
+
+set +e
+NYASH_ENABLE_USING=1 NYASH_USING_STRICT=1 "$BIN" --backend vm "$TMP" >/tmp/nyash-using-strict.out 2>&1
+rc=$?
+set -e
+
+if [ $rc -ne 0 ]; then
+  echo "PASS: strict path missing fails (rc=$rc)" >&2
+else
+  echo "FAIL: strict path missing did not fail" >&2
+  cat /tmp/nyash-using-strict.out >&2 || true
+  exit 1
+fi
+
+echo "All PASS" >&2
+