refactor(tests): restructure suite — 15 files to 8, consolidate parity
- Add conftest.py: shared paths, tolerances, skip markers, fixtures
- Consolidate imports/shapes/smoke/forward → test_model_contract.py
- Consolidate all parity (decoder/refiner/backbone/e2e) → test_parity.py
- Fold 2048 slow test into test_engine.py
- Rework test_conversion.py to use public convert_checkpoint() API
- Simplify test_weights.py: drop CLI/env var tests, keep checksum+config
- Rename tiling/compilation test files for consistency
- Delete 10 redundant files
80 tests collected (75 pass, 4 skip, 1 deselected)
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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