revert(config): rollback tier C a qwen-2.5-72b-instruct (qwen3-235b inferiore)

Run controllo phase2-qwen25-control-001 (seed 42, stessa pipeline Phase 2,
solo tier C switched) ha dimostrato che qwen-2.5-72b è qualitativamente
SUPERIORE a qwen3-235b sul nostro workload:

| metrica           | qwen3-235b | qwen-2.5-72b | delta |
| ----------------- | ---------- | ------------ | ----- |
| max fitness       | 0.0238     | 0.0311       | +30%  |
| median > 0 in gen | mai        | 4 gen su 10  | --    |
| entropy media     | 0.199      | 0.85         | 4.3x  |
| genomi fit > 0    | 5          | 10           | 2x    |
| parse success     | 97.7%      | 100%         | +     |
| durata            | 50 min     | 28 min       | 0.56x |
| LLM calls         | 148        | 90           | 0.61x |
| cost USD          | 0.0223     | 0.0122       | 0.55x |

Controintuitivo: 235B con context 262k era atteso superiore al 72B legacy.
In pratica qwen3-235b in tier C produce strategie meno diverse,
meno parsabili e meno ottimizzabili dal GA.

Ripristinati prezzi cost_tracker tier C a 0.40/0.40 (qwen-2.5-72b).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/unit/test_config.py

@@ -76,6 +76,6 @@ def test_settings_llm_model_defaults(monkeypatch: pytest.MonkeyPatch) -> None:
     assert s.llm_model_tier_s == "google/gemini-3-flash-preview"
     assert s.llm_model_tier_a == "deepseek/deepseek-v4-flash"
     assert s.llm_model_tier_b == "deepseek/deepseek-v4-flash"
-    assert s.llm_model_tier_c == "qwen/qwen3-235b-a22b-2507"
+    assert s.llm_model_tier_c == "qwen/qwen-2.5-72b-instruct"
     assert s.llm_model_tier_d == "openai/gpt-oss-20b"
     assert s.openrouter_base_url == "https://openrouter.ai/api/v1"

tests/unit/test_cost_tracker.py

@@ -4,7 +4,7 @@ from multi_swarm.llm.cost_tracker import CostTracker, estimate_cost
 
 def test_estimate_cost_tier_c():
     cost = estimate_cost(input_tokens=1_000_000, output_tokens=1_000_000, tier=ModelTier.C)
-    assert cost == 0.071 + 0.10
+    assert cost == 0.40 + 0.40
 
 
 def test_estimate_cost_tier_b():