Abort mark-synced on a repeated non-transient 4xx

[grimicorn-agent]

A 422 from MAX_UPDATE_BATCH_SIZE drift or malformed-payload 400 is not systemic, so every chunk fires and fails identically; abort after the first such 4xx. (evidence: src/libs/records.ts markSyncedChunk)

Acceptance criteria

• A 422 from MAX_UPDATE_BATCH_SIZE drift or malformed-payload 400 is not systemic, so every chunk fires and fails identically; abort after the first such 4xx.
• Includes tests covering the change.
• Lint, tests, and CI green.

Promoted from improvement digest #132