fix: quant selector quality-order assertion (closes #139, closes #138)

The test asserted strictly descending bits_per_channel across ALL levels,
but q4_0 (4.0 bits) is a standard fallback that comes after turbo2 (1.5 bits)
despite having more bits. These are different quantization schemes:
TurboQuant vs standard GGUF.

Fix: separate the assertion into two checks:
1. TurboQuant levels (turbo4 > turbo3 > turbo2) must have strictly
   descending bits_per_channel
2. Standard fallback(s) must come after all TurboQuant levels in the list

tests/test_quant_selector.py

@@ -20,9 +20,26 @@ 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
+        """Levels should be ordered from best quality to most aggressive.
+
+        TurboQuant levels (turbo4, turbo3, turbo2) are ordered by descending
+        bits_per_channel (higher = better quality). The standard fallback q4_0
+        comes last regardless of its bits value — it's a different quantization
+        scheme (standard vs TurboQuant) and serves as a last-resort fallback.
+        """
+        # TurboQuant levels must be strictly descending in bits
+        turbo_levels = [l for l in QUANT_LEVELS if l.kv_type.startswith("turbo")]
+        for i in range(len(turbo_levels) - 1):
+            assert turbo_levels[i].bits_per_channel > turbo_levels[i + 1].bits_per_channel
+
+        # Standard fallback(s) must come after all TurboQuant levels
+        standard_levels = [l for l in QUANT_LEVELS if not l.kv_type.startswith("turbo")]
+        if standard_levels:
+            last_turbo = turbo_levels[-1]
+            for std in standard_levels:
+                turbo_idx = QUANT_LEVELS.index(last_turbo)
+                std_idx = QUANT_LEVELS.index(std)
+                assert std_idx > turbo_idx, f"{std.name} should come after all TurboQuant levels"
 
     def test_all_levels_have_required_fields(self):
         for level in QUANT_LEVELS: