New sync-dead-token suite mirrors the Google Health parity test: the three
getValidToken null-return paths each register on the hard-fail ledger, a
successful refresh leaves it empty, and a dead-token cycle returns
{ imported: 0, failed: true } without stamping markSynced or recordSyncSuccess.
Extends the upsert suite with the two write-catch labels (measurements:create,
measurements:update) and the sync orchestrator suite with a write-label
watermark hold. Rewrites the backfill suite to mirror google-health-backfill:
a clean run stamps, a failed verdict throws without stamping, a parked
connection returns without running the sync.
