The test `test_levels_ordered_by_quality` asserted strictly descending
`bits_per_channel`, but `q4_0` (4.0 bits) is a non-TurboQuant fallback
placed last regardless of bit width. The design invariant is:
- TurboQuant levels (turbo4→turbo2): ordered by compression_ratio
ascending (more aggressive = more compression)
- Fallback levels (q4_0): placed after all TurboQuant levels as safe
defaults, not part of the quality progression
Changes:
- `test_levels_ordered_by_quality`: Now validates compression_ratio
ordering for TurboQuant levels only, not across fallbacks
- `test_fallback_quant_is_last`: New test ensuring non-TurboQuant
fallbacks always appear after TurboQuant levels
Closes#138Closes#139 (duplicate)