36ad523 made a manifest file missing from the checkout a 'Source mismatch',
which also broke the tests' manifest-selection helper: it reused
verify_source with hashing neutralised but still read the (renamed) files.
run_refined_dfg.source_manifest(collision, observation_mode) now owns the
frozen-manifest selection; verify_source uses it, then verifies the
checkout. The test helper calls source_manifest directly; source identity
stays owned by test_source_pin. Same errors, manifests and pins.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>