feat(tilemap): typed ObjectLayer schema — ObjectKind + generic accessors (B2, D3)

[Exoridus]

B2 — typed ObjectLayer schema (v0.14 wave 1, D3)

Opt-in, type-safe object-layer API. Fully additive — untyped usage and query() are unchanged.

• ObjectKind is now an as const object (enum-style DX, wire-compatible with Tiled strings, no TS enum); TileMapObjectKind kept as a structural alias.
• ObjectLayer<S extends ObjectSchema = ObjectSchema> with typed accessors: byType(type) (properties typed as S[type]), byKind(kind) (narrowed by the geometry discriminant), where(type, predicate). Default type param reproduces the old untyped behaviour, so getObjectLayer('x') keeps working.
• getObjectLayer<S>(name) threads the schema type param through.

interface LevelObjects { spawn: { team: 'red' | 'blue' }; pickup: { item: 'coin'; amount: number } }
const e = map.getObjectLayer<LevelObjects>('Entities');
e.byType('spawn')[0].properties.team;        // 'red' | 'blue'
e.byKind(ObjectKind.Polygon);
e.where('pickup', o => o.properties.amount > 5);

Schema is a developer promise (Tiled data is untyped at runtime, no runtime validation — documented). Runtime + expectTypeOf compile-time tests added.

Verification (local, all green)

tilemap+tiled typecheck · test suite (3370 passed) · lint:packages · format:check · docs:api:check (in sync)