Update merge test fixtures for the mandatory BacDive correction input/audit contract

Related #1249/#1250. Independent review of frozen W8 found a repository-test
integration failure, not a reason to weaken the new production evidence checks.

BacDiveTransform now declares ec_substrate_correction_policy and
ec_substrate_legacy_mapping as required consumed roles, plus mandatory audit
ec_substrate_corrections.tsv. Two existing inert registered-producer fixture
builders still assume any required role is bacdive_taxon_lookup and only
prepare MediaDive's mandatory audit:

• tests/test_merge_source_freshness.py:88, prepare_source;
• tests/test_merge_upstream_review.py:50, _prepare.

Minimal reproduction at the frozen six-file W8 patch:

PYTHONDONTWRITEBYTECODE=1 PYTHONPATH="$PWD" python -m pytest -q -p no:cacheprovider \
 tests/test_merge_source_freshness.py::test_scoped_ontology_requires_whole_producer_schema_evidence \
 tests/test_merge_upstream_review.py::test_subset_checks_required_upstreams_but_not_unrelated_stale_source

Result: 2 failed, 4 dependency warnings, 8.33 seconds, exit1. Both fail
before their intended merge assertion with:
AssertionError: ... explicit immutable input for ec_substrate_correction_policy.

The new 223-case focused suite does not exercise these fixture branches. Fix
their tiny immutable empty-cohort protocol: consume the actual policy and a
test-owned empty legacy input using the real preparation boundary, write and
record the corresponding header-only producer audit, and preserve finalization/
admission checks. Do not monkeypatch away required roles, forge finalized
receipts, or relax missing/tampered audit rejection. Retain positive evidence
binding checks and missing/unrecorded/tampered negative controls in regressions.

Reviewed source bytes: bacdive.py
a29ccaf97fad660b51299d53a11b76e0f867ee9b91b8a7cdf5f45c616e7aaa4e,
ec_substrate_corrections.py
b6ed3210687739b4399d64889f964115a22b603622fdb293d45ca4fcbc6e9432.
No production run, full suite rerun or implementation edit was made by this
reviewer. Full repository gates remain pending after the fixture repair.