docs: add Homebrew installation

[aaf2tbz]

Summary

Add a first-party Homebrew cask installation path to MetalSharp's README. The cask is maintained in the new aaf2tbz/homebrew-tap and installs the signed arm64 DMG release into /Applications.

Changes

• Add the brew install --cask aaf2tbz/tap/metalsharp installation command.
• Keep source builds available as a clearly labelled developer path.
• Replace the user-facing “Install from Source” documentation label with “Build from Source.”

PR Readiness (MANDATORY)

• Compatibility verified with at least one real game (game + launch method noted below) — documentation-only change; no runtime paths changed.
• No hardcoded paths, secrets, or absolute /Users/... paths introduced
• Config/rules TOML validated if configs/mtsp-rules.toml or DLL maps changed — not applicable; neither changed.
• Version triple (CMakeLists.txt, Cargo.toml, package.json, package-lock.json) in sync if version bumped — not applicable; no version bump.
• Bottle/runtime migration and launch behavior preserved (rollback plan noted if changed) — not applicable; no runtime behavior changed.
• Docs / compatibility matrix updated for user-facing changes
• Regression test added for each bug fix — not applicable; this is not a bug fix.

Local toolchain (run before push)

• git diff --check passes
• Homebrew cask style passes: brew style --cask aaf2tbz/tap/metalsharp
• Homebrew cask audit passes: brew audit --cask --strict aaf2tbz/tap/metalsharp
• Homebrew install resolution passes: brew install --cask --dry-run aaf2tbz/tap/metalsharp
• Rust/C++/TypeScript/Shell project gates — not run; this PR changes only README.md.

Test notes

Validated the companion cask against the published v0.56.8 DMG digest. Homebrew resolves the cask as arm64/macOS and identifies MetalSharp.app as its artifact. No game launch was performed because this PR contains documentation only.

Risk

Low. This changes only installation documentation. Roll back by reverting the README commit; the existing DMG release path remains unchanged.