test: open stores saved by v1.4 and v1.5 with today's schema - #109

Problem

The app opens its store with try! (NormalApp.swift), so a store that can't migrate crashes on every launch. No test opens a store in an older release's format:

• The migration tests open stores with hand-copied model lists, and none includes UsageLimit.
• Key tests create and check a key with the same code, so changing the hash would pass them while breaking every existing key.

Change

• StoreUpgradeTests holds frozen copies of the v1.4 and v1.5 models: stored properties, enum raw values, and a key salt and hash computed outside the app. Each test writes a store with the old models and reopens it with the app's schema.
  • v1.5 is the only release since v1.4 that changed stored properties. A v1.5 store matches v1.6 and v1.7.
• NormalApp.models is the one model list. The app and every test container use it, replacing 5 copies in the tests.

No behavior change.

Tests

• Full unit suite on iPhone 17, iOS 27: 879/879.
• The 5 changed suites on iPhone 15, iOS 17.5: 16/16.
• Each mutation below fails the new tests: