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/workflows/core13-pure.yml

@@ -0,0 +1,27 @@
+name: Core-13 Pure CI
+
+on:
+  push:
+  pull_request:
+
+jobs:
+  test-core13-pure:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Install Rust (stable)
+        uses: dtolnay/rust-toolchain@stable
+
+      - name: Cache cargo
+        uses: Swatinem/rust-cache@v2
+
+      - name: Build (release)
+        run: cargo build --release
+
+      - name: Run tests with Core-13 pure mode
+        env:
+          NYASH_MIR_CORE13_PURE: "1"
+        run: cargo test --all-targets --no-fail-fast
+