Add the shared-world Usurper door game

[mpiorowski]

Summary

• Adds Usurper to the Games hub, running the upstream LORD-style BBS game through a dedicated SSH/PTTY host.
• Uses each player’s arcade handle as their game identity and persists one shared multiplayer world across deployments.

Changes

• Adds the late-usurper host with authenticated SSH access, DOOR32 dropfiles, concurrent node leases, CP437-to-UTF-8 output, and protected function-key filtering.
• Integrates the launcher, session lifecycle, unavailable state, help text, and configuration into late-ssh.
• Builds Usurper from checksum-pinned source, generates its initial world data, and records GPL licensing details.
• Adds Docker, Compose, Kubernetes, persistent-volume, secret, CI, and release workflow support.

Behavior notes

• The host is single-replica and stores the shared world on a persistent volume. Startup only fills missing seed files and clears stale maintenance/online-state files.
• Sessions use a fixed 80×25 game screen. The default host capacity is 10 concurrent nodes.
• The first deployment must use the Usurper-specific deployment workflow because other deployment workflows expect the late-usurper deployment to already exist.

Feature flags

• LATE_USURPER_ENABLED gates access from late-ssh; the standalone host remains deployed when access is disabled.
• LATE_USURPER_SECRET must match between late-ssh and late-usurper.
• Compose and Terraform default the feature to enabled, while the application-level configuration defaults to disabled when unset.

Screenshots / Video

• Not included in this draft; attach before review.

Testing

• Automated: Added coverage for identity derivation, input filtering, launcher state, CP437 conversion, dropfile generation, playname sanitization, node leasing, and non-destructive world seeding. Tests were not run as part of this PR-description draft.
• Manual: Not run; launch, gameplay, disconnect, and restart persistence should be verified against the Compose-hosted service before review.