feat: projectile system for throwing elements (Phase 4.5)

F key launches first inventory item toward mouse cursor as projectile.
Projectiles travel at 350px/s, expire after 2s, destroyed on hitting
non-walkable tiles. Color matches element's periodic table color.
13 new tests (198 total).

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

src/ecs/components.ts

@@ -45,3 +45,8 @@ export const Resource = {
   quantity: [] as number[],       // remaining items to collect
   interactRange: [] as number[],  // max interaction distance in pixels
 };
+
+/** Thrown element/compound projectile */
+export const Projectile = {
+  lifetime: [] as number[],  // remaining lifetime in ms (removed at 0)
+};