Add The Long Thaw platform adventure

[jbeshir]

Summary

• add a compact three-region platform adventure centered on carrying and placing a living ember
• use curated CC0 Kenney assets from the assets MCP server for animated characters, enemies, winter terrain, and flame effects
• add keyboard capture, animated dash, warmth/checkpoint progression, procedural music with a toggle, pause/restart controls, and a complete summit ending
• add widget metadata, deployment configuration, attribution, deterministic browser playtests, and production-only validation

Verification

• npm run build:test
• npm run playtest
• npm run build
• npm run validate:production
• visual review of title, gameplay, and completion captures

Review note

A full Demesne appearance-review run was attempted but cancelled after its production-ending capture bot exceeded a reasonable runtime while trying to complete the entire game without test hooks. The bounded local screenshot review caught and fixed HUD path leakage and unsuitable light-mask compositing before this PR.