fix release pipeline: vendored cava source, fftw in CI, runner arch, smoke test

- Vendor cava/cavacore.c + header (MIT, from karlstav/cava) — the FFI build
  referenced cava/cavacore.c which was never committed, so every CI runner
  failed at scripts/build-cavacore.sh and no release was possible.
- build-cavacore.sh: discover libfftw3.a across Homebrew and Debian/Ubuntu
  multiarch paths (FFTW_PREFIX override preserved).
- release.yml: install fftw before building cavacore; run the boot smoke test
  from a bunfig-free dir (the embedded runtime reads the CWD bunfig.toml and
  this repo's preload entry breaks it — 'preload not found'); use
  macos-15-intel for darwin-x64 (macos-latest is arm64).
- Makefile/build.ts: drop the no-op BUN_CONFIG=bunfig.standalone.toml compile
  dance (Bun never honored it; compile output is config-independent); delete
  bunfig.standalone.toml.

cava/LICENSE-cava.txt

@@ -0,0 +1,19 @@
+Copyright (c) 2015 Karl Stavestrand <karl@stavestrand.no>
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.