ghostty: split durable artifact infrastructure

[vaayne]

What

• pin Mori's Ghostty submodule to the remux fork and build one verified universal macOS + iOS GhosttyKit.xcframework
• add a source-controlled lock, strict contract parser, package manifest, framework-tree hashing, and adversarial fixtures
• make macOS releases draft-first and add protected, no-overwrite GhosttyKit publishing plus a non-publishing release preflight

Why

GhosttyKit production and app release are separate concerns. Actions cache remains acceleration only; it is not durable artifact authority. Landing this infrastructure separately keeps the MoriRemote rewrite review focused and lets the publisher workflow exist on the default branch before its first guarded dispatch.

Deliberate limits

• ghosttykit-lock.json remains a candidate with both artifact digests null until the first protected publication completes.
• Repository immutable releases are still disabled. The publishing path fails closed until draft preflight passes and immutability is enabled.
• This PR does not yet switch ordinary CI, local development, or app release consumers from same-run source builds to durable downloads.
• App-specific MoriRemote runtime and UI changes are excluded.

Verification

• bash -n for all changed shell entry points
• parsed changed workflow YAML and mise.toml
• bash scripts/tests/test-ghosttykit-contract.sh
• bash scripts/tests/test-release-workflows.sh
• git diff --check

A local clean universal build reaches Zig but fails at the known local Zig 0.15.2 macOS linker issue (_sigaction, _waitpid, etc.); the canonical macos-15 workflow build is the artifact gate.