Two follow-ups flagged by code review on Tasks 11/12:
- Lock the back-compat contract for CentralAuditWriter's optional
`nodeIdentity = null` ctor parameter with two explicit tests
(`WriteAsync_PassesThroughCallerSourceNode_WhenNoProviderInjected` and
`WriteAsync_LeavesSourceNodeNull_WhenNoProviderInjected`). The previous
null-provider path was only exercised incidentally via legacy
CentralAuditWriterTests setups; the new tests make the contract explicit
and distinct from the "provider supplied, returns null" path.
- Document why the catch-block log references `evt` rather than the
post-stamp record: the three logged fields (EventId, Kind, Status) are
immutable across the filter+stamp chain, so referencing either name is
equivalent — but the comment warns future maintainers to switch names if
they ever add a field the chain mutates (e.g. SourceNode).
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