v2.3.1

feat: findable in-game mailing-list signup + v2.3.1 (D238)

The endpoint was fine; the feature was invisible and silent:
- the reply was never polled, so success and failure looked identical
- the POST only fired inside the register-200 branch, so a taken name
  silently discarded a valid address
- the only surface was an optional field behind N on the pilot-name screen

New standalone mailing_list screen (MAILING LIST button beside LEADERBOARD,
M as its keyboard twin), both paths now poll and report, and signup rides the
player's confirmation rather than the rename's result.

Verified: zero-warning build, 8/8 suites + a new contract test, canary
unmoved, gate green, windowed screenshot of the live screen.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YRbTvp64hekcJP3NVHukEU