Homebrew bottle: binary install + clean mlx-c dependency handling

[Pushkinist]

Problem

Installing rMLX via Homebrew compiles from source: the formula is
depends_on "rust" => :build + cargo install. So brew install rmlx
pulls the Rust toolchain and builds the whole workspace on the user's Mac
(slow, needs a compiler). The prebuilt binary on the GitHub Release is the
only true binary path, and it's a separate manual download.

We already link mlx-c correctly: the binary records the absolute opt paths

/opt/homebrew/opt/mlx-c/lib/libmlxc.dylib
/opt/homebrew/opt/mlx/lib/libmlx.dylib

and /opt/homebrew is the fixed Homebrew prefix on Apple Silicon, so a
prebuilt binary already resolves mlx-c on any standard arm64 brew machine.
The auto-link is already solved — what's missing is a binary install
channel and a clean "mlx-c not installed" failure.

What to do

Ship a Homebrew bottle (precompiled binary) for the formula.

Then brew install rmlx:

1. Downloads the prebuilt binary — no Rust toolchain, no source compile.
2. Resolves depends_on "mlx-c" → brew installs/links mlx-c automatically.
3. If mlx-c cannot be provided, brew fails cleanly at install (dependency
   error), instead of a cryptic dyld crash at first run.
4. The binary's absolute opt path resolves because brew placed mlx-c at
   exactly /opt/homebrew/opt/mlx-c.

Scope:

• Build the bottle locally (brew bottle) — arm64 only, per supported
  macOS major. (Hosted CI can't build rMLX: no Metal — see
  docs/RELEASING.md. Bottle is built + uploaded as part of the local
  release flow, like the existing tarball.)
• Add the bottle do … sha256 … end block to
  packaging/homebrew/rmlx.rb and sync to the tap.
• Host bottle artifacts (GitHub Release assets or a bottle root) and
  point the formula at them.
• Add the bottle build + upload steps to docs/RELEASING.md.
• Verify: on a clean machine, brew install rmlx pulls the binary (no
  compile) and runs; uninstall mlx-c first → install fails cleanly.

Why

• "Just install rMLX" → one brew install rmlx, binary download, no
  compiler. Removes the from-source step that's the current install pain.
• "Fail if no mlx-c" → routed through brew's dependency engine (clean error
  at install time) instead of a load-time dyld crash that can't be caught as
  a Rust panic (dyld fails before main() runs).

Rejected alternatives

• Static link — no .a shipped by mlx-c/mlx kegs (dylib-only), and
  mlx.metallib is ~150 MB loaded at runtime by libmlx. Bundling/static
  would have to ship + place 150 MB → violates the single-binary tenet.
• Weak-link + startup NULL-check — gives a pretty error on the bare
  tarball path, but makes every mlx symbol weak-bound (fragile) for marginal
  value once the bottle exists.

[Pushkinist]

Landed via #143 (squash-merged to main): scripts/release/build_bottle.sh + make bottle + the bottle lifecycle documented in docs/RELEASING.md. The committed formula is intentionally left source-build (no bottle do block) so existing tap installs keep working; the block is generated + committed at the next release once the real bottle .tar.gz is uploaded. Remaining work (build/upload the actual bottle, clean-machine verify) is now a documented release-time step.