feat: resource collection with E-key interaction (Phase 4.3)

Mineral veins yield metals (Fe, Cu, Zn, Au, Sn), geysers yield S/H.
Resources spawn as ECS entities with gold/orange dot sprites on tiles.
E-key collects nearest resource in range into inventory. Resources
deplete after collection. Visual feedback text. 12 new tests (174 total).

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

src/player/interaction.ts

@@ -0,0 +1,114 @@
+/**
+ * Resource Interaction System — Phase 4.3
+ *
+ * Handles player collecting elements from world resources
+ * (mineral veins, geysers). Press E near a resource to collect.
+ */
+
+import { query } from 'bitecs';
+import type { World } from '../ecs/world';
+import { Position, Resource, PlayerTag } from '../ecs/components';
+import { removeGameEntity } from '../ecs/factory';
+import type { Inventory } from './inventory';
+
+/** Metadata for a resource entity (string data that can't go in bitECS arrays) */
+export interface ResourceInfo {
+  itemId: string;   // element symbol or compound id
+  tileX: number;
+  tileY: number;
+}
+
+/** Result of an interaction attempt */
+export interface InteractionResult {
+  type: 'collected' | 'depleted' | 'inventory_full' | 'nothing_nearby';
+  itemId?: string;
+  remaining?: number;
+}
+
+/** Elements that mineral veins can yield */
+export const MINERAL_ELEMENTS = ['Fe', 'Cu', 'Zn', 'Au', 'Sn'] as const;
+
+/** Elements that geysers can yield */
+export const GEYSER_ELEMENTS = ['S', 'H'] as const;
+
+/**
+ * Deterministic element picker based on tile position and seed.
+ * Same (x, y, seed) always gives the same element.
+ */
+export function pickResourceElement(
+  tileX: number,
+  tileY: number,
+  seed: number,
+  options: readonly string[],
+): string {
+  // Multiplicative hash for spatial distribution
+  const hash = ((tileX * 73856093) ^ (tileY * 19349663) ^ (seed * 83492791)) >>> 0;
+  return options[hash % options.length];
+}
+
+/**
+ * Interaction system — handles E-key resource collection.
+ *
+ * Finds closest resource in range, adds to inventory, decrements quantity.
+ * Returns null if E not pressed, or InteractionResult describing what happened.
+ */
+export function interactionSystem(
+  world: World,
+  justPressedInteract: boolean,
+  inventory: Inventory,
+  resourceData: Map<number, ResourceInfo>,
+): InteractionResult | null {
+  if (!justPressedInteract) return null;
+
+  // Find player position
+  const players = query(world, [Position, PlayerTag]);
+  if (players.length === 0) return null;
+  const playerEid = players[0];
+  const px = Position.x[playerEid];
+  const py = Position.y[playerEid];
+
+  // Find closest resource in range
+  let closestEid: number | null = null;
+  let closestDist = Infinity;
+
+  for (const eid of query(world, [Position, Resource])) {
+    const dx = Position.x[eid] - px;
+    const dy = Position.y[eid] - py;
+    const dist = Math.sqrt(dx * dx + dy * dy);
+    const range = Resource.interactRange[eid];
+
+    if (dist <= range && dist < closestDist) {
+      closestEid = eid;
+      closestDist = dist;
+    }
+  }
+
+  if (closestEid === null) {
+    return { type: 'nothing_nearby' };
+  }
+
+  const info = resourceData.get(closestEid);
+  if (!info) return { type: 'nothing_nearby' };
+
+  // Try to add to inventory
+  const added = inventory.addItem(info.itemId, 1);
+  if (added === 0) {
+    return { type: 'inventory_full', itemId: info.itemId };
+  }
+
+  // Decrement resource quantity
+  Resource.quantity[closestEid] -= 1;
+
+  if (Resource.quantity[closestEid] <= 0) {
+    // Resource depleted — remove entity
+    removeGameEntity(world, closestEid);
+    resourceData.delete(closestEid);
+    return { type: 'depleted', itemId: info.itemId };
+  }
+
+  return {
+    type: 'collected',
+    itemId: info.itemId,
+    remaining: Resource.quantity[closestEid],
+  };
+}