feat(game): stream validation for game metadata

[Seungpyo1007]

What & why

Promotes games to a first-class validated data type without changing TechEngine.

• validates game title, verified, rating/playtime counts, collection fields, ESRB values, image URLs, slug uniqueness, and deterministic shard paths;
• adds python -m app.validate --category game, which walks the million-record game dataset without retaining JSON records in memory;
• documents the game layout and sparse checkout workflow; and
• adds focused unit coverage for valid records, invalid metadata, and shard placement.

Source

Not applicable: this is validator and repository-structure work. The fields are aligned with the existing TechEngine Game model and response schema; no engine code or submodule pointer is changed.

Checklist

• python -m unittest tests.test_game_validation -v passes locally
• Representative existing game record passes the new rules
• Full python -m app.validate (all categories) is intentionally deferred to CI because this working copy uses sparse checkout
• Full streaming game-dataset scan is in progress against the complete local clone; result will be added here
• Game paths use data/game/<slug[:2]>/<slug>.json
• Game slugs and source URL rules are validated

[Seungpyo1007]

Superseded by #120 because GitHub did not refresh this PR's head SHA after the follow-up validation fix.