feat(release): Homebrew bottle build+publish flow (#139)

[Pushkinist]

Summary

Addresses #139 — adds the durable in-repo mechanism to ship a Homebrew bottle (precompiled binary) so a future brew install rmlx downloads the binary instead of compiling from source, while depends_on "mlx-c" keeps the clean install-time dependency-error path (vs a dyld crash).

Approach (non-breaking)

The committed formula packaging/homebrew/rmlx.rb is left unchanged — no bottle do block. Rationale: a block with placeholder sha256 makes brew install try to fetch a nonexistent bottle and hard-fail. With no block, Homebrew falls back to the existing source build (depends_on "rust" => :build + cargo install) for everyone until a real bottle is published. The bottle do block is produced + committed at release time, after the real .tar.gz asset is live on the GitHub Release. This is standard third-party-tap practice.

Changes

• scripts/release/build_bottle.sh (new) — runs brew bottle --json --root-url=<github-release-root> --no-rebuild --force-core-tap rmlx, renames the local double-dash bottle file to the remote single-dash asset name (via the JSON filename field), and prints the ready-to-paste bottle do … end block (root_url + cellar + sha256) plus the exact gh release upload command. shellcheck-clean.
• Makefile — make bottle target (+ .PHONY), slotted after release-sign.
• docs/RELEASING.md — new step in the cut-a-release flow covering the full bottle lifecycle (build keg with --build-bottle, make bottle, upload asset, paste+commit the block, make tap-sync) + a clean-machine bottle verification note.

Verification

• ruby -c packaging/homebrew/rmlx.rb → Syntax OK
• brew style packaging/homebrew/rmlx.rb → no offenses
• brew audit --strict → exit 0, no errors
• shellcheck scripts/release/build_bottle.sh + bash -n → clean
• Source-install path unchanged (formula diff is empty) → existing tap users unaffected.

Remaining (release-time, requires a release cut — not automatable here)

• Build the real bottle (brew install --build-bottle … then make bottle), upload the .tar.gz to the Release, commit the printed bottle do block to the formula, make tap-sync.
• Clean-machine verify: brew install rmlx pulls the binary (no compile); uninstall mlx-c → install fails cleanly.

Note: build_bottle.sh invokes brew bottle rmlx (bare keg name), so the brew bottle --json top-level key is rmlx — matching the jq lookups in the script.

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