Fix duplicate receipt IDs in provenance export

[Haserjian]

Context

test_provenance_export.py::TestAssayCrossValidation::test_no_duplicate_receipt_ids fails with:

AssertionError: Duplicate receipt_ids: ['amesh_weave_001', 'amesh_weave_002',
  'amesh_witness_15fdee3a0cf4661a08f88b93', 'amesh_witness_15fdee3a0cf4661a08f88b93']

Two identical witness payloads produce the same witness receipt ID because the ID is derived from the witness hash, which is deterministic for identical inputs.

Impact

• Pre-existing issue, not introduced by recent changes
• Does not affect CI (test not in default workflow path)
• Affects provenance export correctness when multiple witnesses share identical content

Fix direction

Either deduplicate witnesses before export, or include a sequence counter in the witness receipt ID derivation to guarantee uniqueness.