LocalStorageProvider.prototype.isHealthy(): Promise<boolean>
M70c: runtime.fs.stat(root) succeeds — a disk that vanished or a
permission change is a real, common failure.
Also reports false when the root never proved WRITABLE at connect().
A stat-only probe answered up for a root the process could read and not
write, which is exactly the state X8-9 found: uploads failing while health
said everything was fine.