test(mystery): end-to-end playthrough against sample world

src/engine/playthrough.test.ts

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+import { describe, it, expect } from 'vitest'
+import { parse } from './parser'
+import type { ParserContext } from './parser'
+import { dispatch, initialStateFor } from './dispatcher'
+import { world } from '../world'
+import type { GameState } from './types'
+
+function ctxFor(state: GameState): ParserContext {
+  const room = world.rooms[state.location]
+  const visibleNouns: { id: string; aliases: string[] }[] = []
+  for (const itemId of room?.items ?? []) {
+    const item = world.items[itemId]
+    if (item) visibleNouns.push({ id: itemId, aliases: item.names })
+  }
+  for (const inst of state.inventory) {
+    const item = world.items[inst.id]
+    if (item) visibleNouns.push({ id: inst.id, aliases: item.names })
+  }
+  if (room?.encounter) {
+    visibleNouns.push({ id: room.encounter, aliases: [room.encounter] })
+  }
+  return {
+    knownItems: Object.keys(world.items),
+    knownEncounters: Object.keys(world.encounters),
+    visibleNouns,
+    inventoryItemIds: state.inventory.map((i) => i.id),
+    lastNoun: state.lastNoun,
+    awaitingDisambiguation: state.pendingDisambiguation,
+  }
+}
+
+function play(commands: string[]): GameState {
+  let state = initialStateFor(world)
+  for (const cmd of commands) {
+    const parsed = parse(cmd, ctxFor(state))
+    state = dispatch(state, parsed, world).state
+  }
+  return state
+}
+
+describe('playthrough — sample world', () => {
+  it('reaches the rat-gone flag via the canonical command sequence', () => {
+    const state = play([
+      'take letter',
+      'read letter',  // verb is recognized but encounter takes priority elsewhere; here it's a no-op
+      'n',            // foyer → hallway
+      'take lamp',
+      'e',            // hallway → cellar-stair (triggers rat encounter)
+      'attack rat',
+    ])
+    expect(state.flags['ratGone']).toBe(true)
+    expect(state.location).toBe('cellar-stair')
+    expect(state.encounterState['rat']).toBeUndefined()
+  })
+
+  it('handles invalid moves gracefully', () => {
+    const state = play([
+      'go up',        // foyer has no up exit
+      'n',
+      's',
+      'flibbertigibbet',  // unknown verb
+    ])
+    expect(state.location).toBe('foyer')
+  })
+})