Remove the global validator authority keypair

@snawaz is already working on this.

Since Sep 1, 2026.

## Outcome

Remove the global validator authority keypair after scheduling no longer depends on it.

## Scope

Blocked by #1547, which moves `ScheduledCommitSent` pre-signing out of Magic Program while preserving signature correlation and authorization.

After #1547 is complete:

- delete the global `VALIDATOR_AUTHORITY` static and `validator_authority()` accessor;
- remove production initialization that copies `engine.signer()` into Magic Program;
- reshape test/dev helpers so they set only the thread-local authority pubkey needed by builtin checks;
- keep `engine.signer()` as the only owner and source of the validator keypair.

Programs must continue using `nucleus::tls::AUTHORITY` for validator pubkey checks. Do not weaken any validator-signed instruction or authority check.

## Acceptance criteria

- [ ] #1547 is complete.
- [ ] MBV has no global or static validator `Keypair`.
- [ ] Magic Program does not expose or retain the validator keypair.
- [ ] Only Engine owns and signs with the validator keypair.
- [ ] Existing validator authority checks continue to use the Engine-provided thread-local pubkey.