feat: player entity with WASD movement, tile collision, camera follow (Phase 4.1)

Player spawns at walkable tile near map center. WASD controls movement
(150px/s, normalized diagonal). Tile collision with wall-sliding prevents
walking through acid pools, crystals, geysers. Camera follows player with
smooth lerp. 39 new tests (134 total).

Co-authored-by: Cursor <cursoragent@cursor.com>

src/ecs/components.ts

@@ -34,3 +34,8 @@ export const Health = {
 export const ChemicalComposition = {
   primaryElement: [] as number[],  // atomic number (e.g. 11 for Na)
 };
+
+/** Tag component — marks entity as the player (no data, identity only) */
+export const PlayerTag = {
+  _tag: [] as number[],
+};