feat(app): link check (duplicates, exit target resolution, zone prefix match)

[berkeleynerd]

Add an application-level validation pass after schema validation.

Rules

• Duplicate room IDs are rejected.
• Each exit <to> must resolve to an existing room ID (allow cross-zone exits).
• Room ID must be of the form zone:room and its zone prefix must match the containing <zone><name>; mismatch is an error.

Acceptance

• On any violation, print a precise error and exit non-zero.