BReg evidence export: behaviorRevision omits the engine version and churns on unprojected columns

The export manifest's `behaviorRevision` (`crates/registry-breg/src/evidence_source.rs`) is the digest of the canonical behaviour document. The review found two consequences:

- It carries no engine or protocol version, so a lookup change on the engine side leaves every fingerprint unchanged.
- A change to a stored column the export does not project still churns the revision.

Decide what the revision is meant to fingerprint (the projected contract as served by a named engine version is the natural reading), then either fold the engine version and the projection scope into the digest input, or document the current meaning in `products/evidence/reference/authoring-projects/SOURCE-EXPORT.md` so an operator knows when a changed revision matters.

Origin: review of PR #865.

Start with crates/registry-breg/src/evidence_source.rs and the export manifest’s behaviorRevision digest, then read products/evidence/reference/authoring-projects/SOURCE-EXPORT.md for the documented contract. Define whether the fingerprint covers the projected contract and named engine or protocol version; done when the digest inputs reflect that scope or the documentation clearly states the current meaning and when revisions matter.

Written by the indexing model from the issue text.