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.

build.ts

@@ -5,9 +5,8 @@ import { plugin } from "bun";
 
 // Register the solid transform globally (dedup'd by name). This is what makes
 // `--compile` work: compile-mode builds only apply `onLoad` transform plugins
-// that are registered via `plugin()`, not the `plugins:` array. The compiled
-// binary is then built against an empty bunfig (PODTUI_COMPILE config) so the
-// runtime bakes NO preload — the solid transform is already in the binary.
+// that are registered via `plugin()`, not the `plugins:` array. The transform
+// is fully embedded in the compiled binary.
 plugin(solidPlugin);
 
 const COMPILE =