feat: add Homebrew Cask distribution (stable + nightly)

[jatinkrmalik]

Summary

Adds Homebrew Cask support so users can install VocaMac with brew install --cask vocamac (stable) or brew install --cask vocamac-nightly (nightly builds).

What's included

File	Purpose
homebrew/Casks/vocamac.rb	Stable cask — versioned, livecheck, auto-updated on release
homebrew/Casks/vocamac-nightly.rb	Nightly cask — version :latest, points to stable-named nightly DMG
homebrew/README.md	Tap repo README (for jatinkrmalik/homebrew-vocamac)
.github/workflows/update-homebrew-cask.yml	Auto-updates tap cask on release publish (version + sha256)
.github/workflows/nightly.yml	Modified to also upload stable-named VocaMac-nightly-arm64.dmg alongside dated artifacts
docs/HOMEBREW.md	Comprehensive Homebrew setup & maintenance guide
docs/ARCHITECTURE.md	Updated distribution strategy (planned → shipped)
docs/RELEASE.md	Added Homebrew tap to release checklist

How it works

Stable release:

1. Human publishes a GitHub Release (draft created by release.yml)
2. update-homebrew-cask.yml triggers → downloads DMG → computes sha256 → pushes updated cask to jatinkrmalik/homebrew-vocamac
3. Users run brew upgrade --cask vocamac

Nightly:

1. nightly.yml builds daily → now also uploads VocaMac-nightly-arm64.dmg (stable name) alongside dated VocaMac-nightly-20260606-abc1234-arm64.dmg
2. Cask uses version :latest + sha256 :no_check — no auto-update workflow needed
3. Users run brew reinstall --cask vocamac-nightly to get latest

Conflicts: Both casks install to /Applications/VocaMac.app, so they declare conflicts_with each other.

Setup required (already done)

• Tap repo created: jatinkrmalik/homebrew-vocamac
• HOMEBREW_TAP_TOKEN secret added to vocamac repo
• First stable release published (so the DMG URL exists)
• Copy homebrew/Casks/*.rb and homebrew/README.md to the tap repo (one-time)

Future: submit to homebrew-cask

Once VocaMac hits ~75 stars, the cask can be submitted to Homebrew/homebrew-cask for brew install --cask vocamac without a tap. See docs/HOMEBREW.md for requirements.