Test screenshot catalog contracts

[cbusillo]

Summary

• replace exact screenshot filenames/counts with semantic catalog contracts
• verify set mappings, display types, existing/unique files, upload caps, and full-platform coverage
• verify the combined iOS catalog composes device-specific sets in upload order
• verify selected_assets preserves requested set and configured asset order
• remove duplicate script-source greps from the workflow test

Evidence

• 143 targeted screenshot/workflow tests passed
• all 10 routine Python lane files passed
• scripts/commit-gate.sh passed, including all 889 Swift tests
• mutation probes failed as expected for missing mappings, duplicate paths, wrong display type, over-cap catalogs, broken iOS composition, and reordered selection
• Opus final review approved
• Gemini final route was invoked but returned no usable review text

Follow-up

The manual workflow currently omits the valid iphone65 set. That pre-existing operator-surface drift is intentionally not mixed into this test-only PR and remains the next focused fix.

Refs #579