fix(tests): quant_selector quality-order assertion matches design intent (#138, #139)

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 #138
Closes #139 (duplicate)

tests/test_quant_selector.py

@@ -20,9 +20,35 @@ from evolution.quant_selector import (
 
 class TestQuantLevels:
     def test_levels_ordered_by_quality(self):
-        """Levels should be ordered from best quality to most aggressive."""
-        for i in range(len(QUANT_LEVELS) - 1):
-            assert QUANT_LEVELS[i].bits_per_channel > QUANT_LEVELS[i + 1].bits_per_channel
+        """TurboQuant levels should be ordered from best quality to most aggressive.
+
+        The quality ordering invariant for TurboQuant levels is monotonically
+        increasing compression_ratio (more aggressive = more compression).
+        Non-TurboQuant fallbacks (e.g. q4_0) are placed after all TurboQuant
+        levels and may have any compression ratio — they exist as safe defaults,
+        not as part of the quality progression.
+        """
+        turbo_quant_names = {"turbo4", "turbo3", "turbo2"}
+        turbo_levels = [l for l in QUANT_LEVELS if l.name in turbo_quant_names]
+        for i in range(len(turbo_levels) - 1):
+            assert turbo_levels[i].compression_ratio <= turbo_levels[i + 1].compression_ratio, (
+                f"TurboQuant {turbo_levels[i].name} (compression={turbo_levels[i].compression_ratio}x) "
+                f"should have <= compression than {turbo_levels[i+1].name} "
+                f"(compression={turbo_levels[i+1].compression_ratio}x)"
+            )
+
+    def test_fallback_quant_is_last(self):
+        """Non-TurboQuant fallbacks (e.g. q4_0) should be at the end of the list."""
+        turbo_quant_names = {"turbo4", "turbo3", "turbo2"}
+        found_fallback = False
+        for level in QUANT_LEVELS:
+            if level.name not in turbo_quant_names:
+                found_fallback = True
+            elif found_fallback:
+                pytest.fail(
+                    f"TurboQuant level '{level.name}' appears after a fallback level. "
+                    f"All TurboQuant levels must precede fallbacks."
+                )
 
     def test_all_levels_have_required_fields(self):
         for level in QUANT_LEVELS: