0733 — Claim observations use xid

Context

The active claim observation holds its own transaction id, but its SQL alias and Go field still called it xmax, the former snapshot boundary. The user approved xid, Xid, and pending_xid as the shorter accurate names.

Decision

Name the observation SQL value xid, its Go field Xid, and the stored cursor column pending_xid. Keep PostgreSQL snapshot xmax terminology where it still describes that actual value, including the regression test. This names the observation mechanism retained by 0715.

Consequences

The pre-v1 table definition, SQL callers, website sandbox, and current docs use the new names together. Existing databases with pending_xmax need their cursor tables migrated or recreated before running this code. No database is changed by this rename. Claim behavior is unchanged.