[Epic] Tilemap support

[stormmuller]

Goal

Add tilemap support to the engine — a common building block for 2D games (platformers, top-down RPGs, puzzle games) that Forge currently has no support for.

Scope (suggested)

• A tilemap component/system for rendering a grid of tiles from a tileset texture (efficient batched rendering, ideally reusing the existing instanced sprite-batching path in src/rendering).
• Support for a common interchange format (e.g. Tiled's .tmx/.json export) so games aren't forced to hand-author tile data.
• Collision integration: a way to generate physics colliders from tile layers (e.g. treating solid tiles as static colliders, akin to how TerrainCollider works for heightmaps).
• Layered tilemaps (background/foreground/collision layers).

Why this is a milestone, not a single issue

This spans rendering (batched tile rendering), asset loading (tileset/map parsing), and physics (tile-derived colliders), and likely needs several sequenced PRs. Break this down into sub-issues as design decisions are made (e.g. "Tiled JSON parser", "tilemap rendering system", "tile-to-collider generation").

Acceptance criteria (top-level)

• A tilemap can be loaded from a standard interchange format and rendered efficiently
• Solid tiles can generate physics colliders
• Documented in documentation-site/docs/docs with a demo under documentation-site/src/pages/demos