Build and smoke-test native arm64 and x86_64 macOS apps

[Infiland]

Child scope of #736. Depends on #854 and #855.

Problem

The release matrix builds only one arm64 macOS app. Both supported architectures need their own native, dependency-verified build and final-package GUI smoke on matching GitHub-hosted hardware before publication logic is allowed to consume them.

Scope

• Keep a native Apple Silicon lane and add the official macos-26-intel x86_64 lane, each with CPython 3.12.10 and the lock proven by Prove and lock the macOS x86_64 dependency baseline #854.
• Give every intermediate artifact and payload an unambiguous architecture-specific name: GM2Godot-macos-arm64 and GM2Godot-macos-x86_64.
• Run the verifier from Verify macOS Mach-O architecture and deployment targets #855 against each source app, ZIP, and DMG before upload.
• Extract each final ZIP on its matching native runner, launch Contents/MacOS/GM2Godot, and require the existing one-use GM2GODOT_GUI_SMOKE_RECEIPT contract within a bounded timeout with deterministic process cleanup.
• Preserve existing Linux and Windows build bytes/names and do not publish a GitHub Release in this child.

Acceptance criteria

• The matrix contains exactly one native arm64 macOS job and one native x86_64 macOS job with matching setup-python architecture and environment receipts.
• ZIP/DMG and uploaded workflow-artifact names are architecture-specific and cannot collide.
• Both jobs pass complete Mach-O/minimum-version verification from Verify macOS Mach-O architecture and deployment targets #855.
• Each final ZIP reaches the GUI event loop on its matching native runner and writes the exact smoke receipt before a bounded clean exit.
• Missing receipt, wrong architecture, early exit, timeout, or cleanup failure blocks artifact upload.
• Linux and Windows build and upload behavior remains unchanged.
• Pyright, Ruff, workflow/smoke tests, and the full unit suite pass.

Non-goals

Do not change release-publisher asset inventories, automatic updater selection, Developer ID signing, notarization, or stapling here.