Add signed bundle installer and recovery

[dovvnloading]

Summary

• add transport-independent signed recipe bundle staging and atomic activation
• persist digest-named generations, the current/previous verified state, and a chained Ed25519 keyring update history
• reject reparse points, hardlinks, path escapes, source mutation, incomplete state, and unverified startup recovery
• require explicit trusted rollback authorization and keep provider/process execution disabled
• add installer ADR/evidence and adversarial crash, tamper, key-rotation, and recovery tests

Scope boundary

This PR does not copy user-owned conversation artifacts, decode images, publish outputs, launch processes, provide a sandbox/provider, or enable execution. The next gate is trusted artifact copy-in/output validation and publication.

Verification

• Python: 171 passed, 1 native-platform skip, 1 pre-existing pytest-asyncio warning
• Frontend: 39 tests passed; lint, typecheck, and production build passed
• compileall, contract generation, and git diff --check passed
• prior native broker PR Add authenticated native broker adapter #39 is already merged into main