Roadmap: manual / DMG install path for non-Homebrew users

[jmcte]

Context

docs/NATIVE_ONLY_REDESIGN.md Phase 5 lists as a deliverable:

add manual install path for non-Homebrew users

Today the only documented install paths in docs/INSTALLATION.md are cargo install (developer-only) and the Homebrew formula. Operators who don't use Homebrew or who need an offline install have no clean route.

Proposed Fix

• Ship a notarized DMG (or .pkg installer) as a release asset alongside the existing tarballs (Include APW.app in release archives #30, fix: address issue #21 #36).
• Document the manual install path in docs/INSTALLATION.md end-to-end: download → verify signature → drag-to-Applications (or installer) → run apw app install if needed.
• Verify Gatekeeper accepts the artifact on a clean macOS host with no developer tools installed.

Acceptance criteria

• Release pipeline (release.yml) produces a notarized DMG or pkg.
• Manual install steps documented with checksum verification and Gatekeeper expectations.
• Smoke-tested on a clean macOS host without brew or Xcode.