Purity / rebuild guarantee test (#30)

The cache is a rebuildable index over gitea, never a source of truth (D4).
Two tests lock that invariant where the durable cache actually lives:

- packages/core: cache-purity-v0.test.ts — file-backed node:sqlite. Build the
  SQLite mirror from a gitea snapshot, capture every re-derived field, delete the
  .sqlite file, rebuild from the same snapshot, assert byte-identical. Plus a
  structural D4 guard: every issues-table column must map to a gitea field, so a
  future user-authored column can't silently break rebuild-ability.

- apps/desktop: snapshot-store.test.ts — the shipped durable cache is the JSON
  snapshot-store. Delete the file → loadSnapshot returns null (degrades to
  no-cache, never throws), which is what forces the next getSnapshot to reconcile
  fresh from gitea. Corrupt/partial files are likewise treated as no-cache.
  Stands up vitest for the desktop main process (first unit tests there);
  electron is mocked, snapshot path is injected.

No native better-sqlite3 shipped: the SQLite mirror has no consumer on any hot
path yet, so wiring it into main (native module + asarUnpack + dmg re-verify)
would add packaging risk for no runtime benefit. The purity invariant is proven
at the seam for both caches; the native driver migration is deferred until
SQLite becomes load-bearing.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/desktop/package.json

@@ -11,6 +11,7 @@
     "build": "electron-vite build",
     "start": "electron-vite preview",
     "typecheck": "tsc --noEmit",
+    "test": "vitest run",
     "e2e": "electron-vite build && playwright test",
     "e2e:only": "playwright test",
     "e2e:report": "playwright show-report e2e/.artifacts/report",