Audit Stamping

Mirrors the in-app Help. The live copy in your console and `dlake guide help` are always current.

Per-table dl_ audit columns (created/modified by/at) maintained by an AFTER trigger (tr_<t>_dl_AuditStamp), attributing writes to the JWT identity (including which API key wrote, via SESSION_CONTEXT('api_key_id')). A table can be frozen (tr_<t>_dl_AuditFreeze): stamps preserved but no longer updated. Stamping columns are server-managed — never send them in write bodies. Enable regenerates and restarts DAB for you (for the new columns, and so DAB sees the stamping trigger — see Triggers and the Data API); if it reports the redeploy FAILED, restart DAB yourself. Disable needs no restart: it swaps the stamping trigger for the freeze trigger, so the table carries a trigger before and after.