feat(mystery): add Zod schemas for markdown frontmatter

Defines runtime validation schemas (roomFrontmatterSchema,
itemFrontmatterSchema, endingFrontmatterSchema, encounterFrontmatterSchema)
and their inferred TypeScript types. All 8 TDD tests pass.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/world/schema.test.ts

@@ -0,0 +1,81 @@
+import { describe, it, expect } from 'vitest'
+import { roomFrontmatterSchema, itemFrontmatterSchema, endingFrontmatterSchema, encounterFrontmatterSchema } from './schema'
+
+describe('roomFrontmatterSchema', () => {
+  it('accepts a fully populated room', () => {
+    const data = {
+      id: 'foyer',
+      title: '[ Foyer ]',
+      exitN: 'hallway',
+      exitS: null,
+      exitE: null,
+      exitW: null,
+      exitU: null,
+      exitD: null,
+      items: ['letter'],
+      encounter: null,
+      safe: true,
+    }
+    expect(() => roomFrontmatterSchema.parse(data)).not.toThrow()
+  })
+
+  it('accepts a locked exit with sibling fields', () => {
+    const data = {
+      id: 'hall',
+      title: '[ Hall ]',
+      exitN: null, exitS: null, exitE: null, exitW: null, exitU: null,
+      exitD: 'vault',
+      exitDRequires: 'rusted-key',
+      exitDLockedText: 'The door is locked.',
+      items: [],
+    }
+    expect(() => roomFrontmatterSchema.parse(data)).not.toThrow()
+  })
+
+  it('rejects a room missing a required exit field', () => {
+    const data = { id: 'r', title: '[ R ]', exitN: null, items: [] }
+    expect(() => roomFrontmatterSchema.parse(data)).toThrow()
+  })
+})
+
+describe('itemFrontmatterSchema', () => {
+  it('accepts an item with state', () => {
+    const data = {
+      id: 'lamp',
+      names: ['lamp', 'oil lamp'],
+      short: 'an oil lamp',
+      takeable: true,
+      initialState: { lit: false },
+    }
+    expect(() => itemFrontmatterSchema.parse(data)).not.toThrow()
+  })
+
+  it('accepts an item without state (defaults to {})', () => {
+    const parsed = itemFrontmatterSchema.parse({
+      id: 'letter',
+      names: ['letter'],
+      short: 'a letter',
+      takeable: true,
+    })
+    expect(parsed.initialState).toEqual({})
+  })
+})
+
+describe('endingFrontmatterSchema', () => {
+  it('accepts true ending shape', () => {
+    const data = { id: 'true', whenFlags: { ratGone: true } }
+    expect(() => endingFrontmatterSchema.parse(data)).not.toThrow()
+  })
+
+  it('rejects unknown ending id', () => {
+    const data = { id: 'mercy', whenFlags: {} }
+    expect(() => endingFrontmatterSchema.parse(data)).toThrow()
+  })
+})
+
+describe('encounterFrontmatterSchema', () => {
+  it('accepts an encounter narration doc', () => {
+    const data = { id: 'rat', startsIn: 'cellar-stair', initialPhase: 'lurking' }
+    expect(() => encounterFrontmatterSchema.parse(data)).not.toThrow()
+  })
+})