test(01-03): add failing tests for Base64 codec + full round-trip [RED]

- round-trip.test.ts (3 tests): full pipeline EXPORT -> IMPORT -> MIGRATE
  -> WRAP -> UNWRAP -> IDB PUT -> IDB GET exercising every save layer
  file end-to-end (CORE-09 + CORE-04 + CORE-06 + CORE-07); plus DoS-cap
  rejection at MAX_IMPORT_BYTES + 1; plus malformed-Base64 rejection

RED phase per TDD plan-level gate. Tests fail because codec.ts does not
exist yet.

src/save/round-trip.test.ts

@@ -0,0 +1,87 @@
+import { describe, it, expect, beforeEach } from 'vitest';
+import 'fake-indexeddb/auto';
+import { wrap, unwrap } from './envelope';
+import { migrate, CURRENT_SCHEMA_VERSION } from './migrations';
+import { exportToBase64, importFromBase64 } from './codec';
+import { openSaveDB } from './db';
+
+// CORE-09 + CORE-04 + CORE-06 + CORE-07: full save round-trip exercising
+// every save layer file end-to-end. This is the load-bearing integration
+// test for Phase 1 — if this passes, Phase 2 can reasonably trust that
+// the save subsystem is wired correctly.
+
+beforeEach(async () => {
+  // Same store-contents reset pattern as the unit tests — see db.test.ts
+  // and snapshots.test.ts for why we don't deleteDatabase.
+  const db = await openSaveDB();
+  for (const store of ['saves', 'save_snapshots'] as const) {
+    const all = await db.getAll(store);
+    for (const e of all) {
+      await db.delete(store, e.id);
+    }
+  }
+});
+
+describe('CORE-09 + CORE-04 + CORE-06 + CORE-07: full save round-trip', () => {
+  it('synthetic v0 envelope migrates, round-trips through Base64, validates, persists', async () => {
+    // Pretend a player had an old v0 save lying around (CONTEXT D-05 synthetic v0).
+    const v0Payload = { garden: [{ id: 'tile-1' }, { id: 'tile-2' }] };
+    // v0 envelope: schemaVersion 0, with a placeholder checksum that we won't
+    // verify (the v0 era didn't have our checksum scheme, but the schema
+    // accepts it because checksum just has to be 8 hex chars).
+    const v0Envelope = {
+      schemaVersion: 0,
+      payload: v0Payload,
+      checksum: '00000000', // 8-char hex placeholder
+    };
+
+    // EXPORT through Base64 codec
+    const exported = exportToBase64(v0Envelope);
+    expect(exported.length).toBeGreaterThan(0);
+
+    // IMPORT (simulating a fresh browser) — note: import returns a parsed
+    // envelope that PASSES our SaveEnvelopeSchema (schemaVersion 0 is allowed
+    // since z.number().nonnegative()).
+    const imported = importFromBase64(exported);
+    expect(imported.schemaVersion).toBe(0);
+
+    // MIGRATE the imported payload
+    const { payload, toVersion } = migrate(imported.payload, imported.schemaVersion);
+    expect(toVersion).toBe(CURRENT_SCHEMA_VERSION);
+    expect(payload).toMatchObject({
+      garden: { tiles: [{ id: 'tile-1' }, { id: 'tile-2' }] },
+      plants: [],
+      harvestedFragmentIds: [],
+      lastTickAt: expect.any(Number),
+      settings: {
+        musicVolume: expect.any(Number),
+        ambientVolume: expect.any(Number),
+        sfxVolume: expect.any(Number),
+      },
+    });
+
+    // WRAP with current version and a valid checksum, UNWRAP to verify
+    const v1Envelope = wrap(payload, toVersion);
+    expect(unwrap(v1Envelope)).toEqual(payload);
+
+    // PERSIST to IDB and read back (CORE-04)
+    const db = await openSaveDB();
+    await db.put('saves', {
+      id: 'main',
+      envelope: v1Envelope,
+      savedAt: new Date().toISOString(),
+    });
+    const retrieved = await db.get('saves', 'main');
+    expect(retrieved?.envelope).toEqual(v1Envelope);
+    expect(unwrap(retrieved!.envelope)).toEqual(payload);
+  });
+
+  it('rejects oversized Base64 import (DoS cap)', () => {
+    const huge = 'A'.repeat(50 * 1024 * 1024 + 1);
+    expect(() => importFromBase64(huge)).toThrow(/exceeds/);
+  });
+
+  it('rejects malformed Base64', () => {
+    expect(() => importFromBase64('not-valid-base64-)(*&^%$')).toThrow();
+  });
+});