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/player/projectile.ts

@@ -0,0 +1,141 @@
+/**
+ * Projectile System — Throw Elements & Compounds
+ *
+ * Player launches projectiles toward mouse cursor.
+ * Projectiles travel in a straight line, expire after a lifetime,
+ * and are destroyed on hitting non-walkable tiles.
+ */
+
+import { addEntity, addComponent, query } from 'bitecs';
+import type { World } from '../ecs/world';
+import { Position, Velocity, SpriteRef, Projectile } from '../ecs/components';
+import { removeGameEntity } from '../ecs/factory';
+import { isTileWalkable } from './collision';
+import { ElementRegistry } from '../chemistry/elements';
+import { CompoundRegistry } from '../chemistry/compounds';
+import type { TileGrid } from '../world/types';
+
+/** Extra data for a projectile entity (string data) */
+export interface ProjectileData {
+  itemId: string;  // element symbol or compound id
+}
+
+// === Constants ===
+
+/** Projectile flight speed in pixels per second */
+export const PROJECTILE_SPEED = 350;
+
+/** Projectile lifetime in milliseconds */
+export const PROJECTILE_LIFETIME = 2000;
+
+/** Projectile visual radius */
+export const PROJECTILE_RADIUS = 5;
+
+// === Direction ===
+
+/**
+ * Calculate normalized direction from source to target.
+ * Returns (1, 0) if source == target (default: right).
+ */
+export function calculateDirection(
+  fromX: number, fromY: number,
+  toX: number, toY: number,
+): { dx: number; dy: number } {
+  const dx = toX - fromX;
+  const dy = toY - fromY;
+  const len = Math.sqrt(dx * dx + dy * dy);
+
+  if (len < 0.001) {
+    return { dx: 1, dy: 0 }; // default direction
+  }
+
+  return { dx: dx / len, dy: dy / len };
+}
+
+// === Launch ===
+
+/**
+ * Get the display color for a chemical item.
+ * Uses element color from registry, or compound color, or default white.
+ */
+function getItemColor(itemId: string): number {
+  const el = ElementRegistry.getBySymbol(itemId);
+  if (el) return parseInt(el.color.replace('#', ''), 16);
+
+  const comp = CompoundRegistry.getById(itemId);
+  if (comp) return parseInt(comp.color.replace('#', ''), 16);
+
+  return 0xffffff;
+}
+
+/**
+ * Create a projectile entity flying from (fromX, fromY) toward (toX, toY).
+ * @returns entity ID
+ */
+export function launchProjectile(
+  world: World,
+  projData: Map<number, ProjectileData>,
+  fromX: number,
+  fromY: number,
+  toX: number,
+  toY: number,
+  itemId: string,
+): number {
+  const dir = calculateDirection(fromX, fromY, toX, toY);
+
+  const eid = addEntity(world);
+  addComponent(world, eid, Position);
+  addComponent(world, eid, Velocity);
+  addComponent(world, eid, SpriteRef);
+  addComponent(world, eid, Projectile);
+
+  Position.x[eid] = fromX;
+  Position.y[eid] = fromY;
+  Velocity.vx[eid] = dir.dx * PROJECTILE_SPEED;
+  Velocity.vy[eid] = dir.dy * PROJECTILE_SPEED;
+  SpriteRef.color[eid] = getItemColor(itemId);
+  SpriteRef.radius[eid] = PROJECTILE_RADIUS;
+  Projectile.lifetime[eid] = PROJECTILE_LIFETIME;
+
+  projData.set(eid, { itemId });
+
+  return eid;
+}
+
+// === System ===
+
+/**
+ * Update projectiles: decrement lifetime, remove expired/collided.
+ * Runs AFTER movementSystem (projectiles moved by generic movement).
+ */
+export function projectileSystem(
+  world: World,
+  deltaMs: number,
+  grid: TileGrid,
+  tileSize: number,
+  walkable: Set<number>,
+  projData: Map<number, ProjectileData>,
+): void {
+  const toRemove: number[] = [];
+
+  for (const eid of query(world, [Position, Projectile])) {
+    // Decrement lifetime
+    Projectile.lifetime[eid] -= deltaMs;
+
+    // Check expiry
+    if (Projectile.lifetime[eid] <= 0) {
+      toRemove.push(eid);
+      continue;
+    }
+
+    // Check tile collision (point check at projectile center)
+    if (!isTileWalkable(Position.x[eid], Position.y[eid], grid, tileSize, walkable)) {
+      toRemove.push(eid);
+    }
+  }
+
+  for (const eid of toRemove) {
+    removeGameEntity(world, eid);
+    projData.delete(eid);
+  }
+}