Repair inert MediaDive test fixtures for the mandatory material-scope audit contract

Observed failure

Found in PR #1239, implementing #1236/#1237; CI run 36554487707.

At PR head 99a35c93c1b5155a6584a5b7823630b5999efae8, both Python 3.10 and 3.12 CI fail with the same mandatory producer-audit exception. Each run reports 17 failed, 13 errors, 4,854 passed, and 56 skipped:

• test_merge_consumed_input_review: 8 failures.
• test_merge_upstream_review: 9 failures.
• test_mediadive_raw_inputs: 8 setup errors.
• test_merge_source_freshness: 5 setup errors.

The local targeted reproduction confirms:

prepare_source(..., "mediadive")
  -> transform.finalize(fresh_run=True)
  -> verify_producer_audits
SourceFinalizationRequired:
  Missing or unexpected producer audit snapshots; rerun the producer

Root cause

The new MediaDive material-scope audit is correctly mandatory, including for zero-cohort producers. Three test setup sites bypass the actual run() method and directly finalize synthetic graph TSVs without creating/recording the new audit: shared prepare_source, upstream _prepare, and raw-input admitted. The shared helper accounts for two affected modules.

The actual MediaDive writer already emits and records the canonical header-only audit when its selected immutable media list is empty. That is the valid fixture protocol; relaxing the runtime contract or manually forging finalized audit evidence would hide the defect.

Requested bounded fix

• Complete these three fake-producer fixtures with the actual MaterialScopeAudit.write() stage over their consumed immutable empty media-list fixture, after all required/optional reads and before finalization.
• Keep reader-only helpers free of implicit audit writes.
• Add explicit regression coverage for valid zero-cohort evidence and rejection when the required audit or its producer-time recording is absent.
• Preserve runtime admission and all existing freshness/raw-input negative assertions without bypasses.

Acceptance

The four affected modules and focused audit/input tests pass. Missing, changed, deleted, or unrecorded required audits remain fail-closed. Complete repository gates and all three supported CI Python versions pass at the revised checkpoint. This is a test-protocol repair, not a change to the reviewed mapping, source replay, or production scientific behavior.