feat(phase-2.5): population_prompt_diversity metric + piano aggiornato

Task 5 del piano Phase 2.5: nuovo modulo src/multi_swarm/metrics/diversity.py
con population_prompt_diversity(prompts) che ritorna la diversità media
1 - SequenceMatcher.ratio() su tutte le coppie distinte. 0.0 identici,
fino a ~0.9 totalmente diversi (SequenceMatcher considera spazi/lunghezza).

5 test: edge case empty/single, identici, diversi, intermediate, simmetria.

Piano aggiornato a stato "IMPLEMENTATO 2026-05-11": checkbox task 1-5
spuntate, task 6 (cost attribution per call_kind) deferito con motivazione.
Header preambolo aggiornato con trigger verificati e decisione collaterale
rollback tier C.

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

tests/unit/test_diversity.py

@@ -0,0 +1,45 @@
+from __future__ import annotations
+
+from multi_swarm.metrics.diversity import population_prompt_diversity
+
+
+def test_empty_or_single_prompt_zero_diversity() -> None:
+    assert population_prompt_diversity([]) == 0.0
+    assert population_prompt_diversity(["solo prompt"]) == 0.0
+
+
+def test_identical_prompts_zero_diversity() -> None:
+    prompts = ["Strategia RSI < 30 long"] * 5
+    assert population_prompt_diversity(prompts) == 0.0
+
+
+def test_completely_different_prompts_high_diversity() -> None:
+    prompts = [
+        "AAAAAA AAAA AAAAA",
+        "BBBBBB BBBB BBBBB",
+        "CCCCCC CCCC CCCCC",
+        "DDDDDD DDDD DDDDD",
+    ]
+    d = population_prompt_diversity(prompts)
+    # SequenceMatcher considera spazi e lunghezza simili → similarity > 0
+    # anche su stringhe completamente "diverse". Soglia realistica: 0.8.
+    assert d > 0.8
+
+
+def test_partial_overlap_intermediate_diversity() -> None:
+    prompts = [
+        "Strategia momentum 1h con RSI",
+        "Strategia momentum 1h con SMA",
+        "Strategia momentum 4h con RSI",
+    ]
+    d = population_prompt_diversity(prompts)
+    assert 0.05 < d < 0.5
+
+
+def test_diversity_symmetric() -> None:
+    prompts_a = ["x", "yy", "zzz"]
+    prompts_b = ["zzz", "x", "yy"]
+    assert (
+        abs(population_prompt_diversity(prompts_a)
+            - population_prompt_diversity(prompts_b)) < 1e-9
+    )