feat(core): compute_adaptive_threshold pure function + tests

Implementa il calcolo del percentile rolling con warmup,
transizione min_days → target_days e floor assoluto. Pure
function senza I/O: il caller passa la sequenza pre-filtrata
(NULL e fetch_ok=0 esclusi).

Tests: warmup, transizione finestra, floor, percentili.

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

tests/unit/test_adaptive_threshold.py

@@ -0,0 +1,142 @@
+"""TDD per :mod:`cerbero_bite.core.adaptive_threshold`.
+
+Spec: ``docs/superpowers/specs/2026-05-08-iv-rv-adaptive-gate-design.md``.
+"""
+
+from __future__ import annotations
+
+from decimal import Decimal
+
+import pytest
+
+from cerbero_bite.core.adaptive_threshold import compute_adaptive_threshold
+
+
+# ---------------------------------------------------------------------------
+# Warmup
+# ---------------------------------------------------------------------------
+
+
+def test_empty_history_returns_none() -> None:
+    out = compute_adaptive_threshold(
+        history=[],
+        percentile=Decimal("0.25"),
+        absolute_floor=Decimal("0"),
+        min_days=30,
+        target_days=60,
+    )
+    assert out is None
+
+
+def test_history_under_one_day_returns_none() -> None:
+    out = compute_adaptive_threshold(
+        history=[Decimal("3")] * 50,  # 50 tick < 96
+        percentile=Decimal("0.25"),
+        absolute_floor=Decimal("0"),
+        min_days=30,
+        target_days=60,
+    )
+    assert out is None
+
+
+def test_history_exactly_one_day_returns_percentile() -> None:
+    history = [Decimal(i) / Decimal("10") for i in range(96)]  # 0.0..9.5
+    out = compute_adaptive_threshold(
+        history=history,
+        percentile=Decimal("0.25"),
+        absolute_floor=Decimal("0"),
+        min_days=30,
+        target_days=60,
+    )
+    # P25 di [0.0..9.5] passo 0.1 con method='linear': k=23.75, val ≈ 2.375
+    assert out is not None
+    assert Decimal("2.3") < out < Decimal("2.5")
+
+
+def _ramp(n: int, base: Decimal = Decimal("1")) -> list[Decimal]:
+    """Ramp lineare 1, 2, 3, ... per testare in modo predicibile il P25."""
+    return [base * Decimal(i + 1) for i in range(n)]
+
+
+def test_below_min_days_uses_full_history() -> None:
+    # 5 giorni di storia (5*96=480 tick), min_days=30, target=60.
+    # Window = full history.
+    history = _ramp(480)
+    out = compute_adaptive_threshold(
+        history=history,
+        percentile=Decimal("0.25"),
+        absolute_floor=Decimal("0"),
+        min_days=30,
+        target_days=60,
+    )
+    # P25 di ramp [1..480] = ~120.75 (k=479*0.25=119.75, ramp[119]=120, ramp[120]=121)
+    assert out is not None
+    assert Decimal("120") <= out <= Decimal("121")
+
+
+def test_between_min_and_target_uses_min_window() -> None:
+    # 50 giorni di storia (4800 tick), min_days=30, target=60. Window = ultimi 30g.
+    history = _ramp(4800)  # values 1..4800
+    out = compute_adaptive_threshold(
+        history=history,
+        percentile=Decimal("0.25"),
+        absolute_floor=Decimal("0"),
+        min_days=30,
+        target_days=60,
+    )
+    # Window = ultimi 30*96=2880, valori 1921..4800, P25 ≈ 2640
+    assert out is not None
+    assert Decimal("2630") <= out <= Decimal("2650")
+
+
+def test_above_target_uses_target_window() -> None:
+    # 100 giorni (9600 tick), target=60. Window = ultimi 60g.
+    history = _ramp(9600)  # values 1..9600
+    out = compute_adaptive_threshold(
+        history=history,
+        percentile=Decimal("0.25"),
+        absolute_floor=Decimal("0"),
+        min_days=30,
+        target_days=60,
+    )
+    # Window = ultimi 5760, valori 3841..9600, P25 ≈ 5280
+    assert out is not None
+    assert Decimal("5270") <= out <= Decimal("5290")
+
+
+def test_floor_binding_overrides_low_percentile() -> None:
+    history = [Decimal("0.5")] * 200
+    out = compute_adaptive_threshold(
+        history=history,
+        percentile=Decimal("0.25"),
+        absolute_floor=Decimal("3"),
+        min_days=30,
+        target_days=60,
+    )
+    assert out == Decimal("3")
+
+
+def test_floor_not_binding_returns_percentile() -> None:
+    history = [Decimal("5")] * 200
+    out = compute_adaptive_threshold(
+        history=history,
+        percentile=Decimal("0.25"),
+        absolute_floor=Decimal("0"),
+        min_days=30,
+        target_days=60,
+    )
+    assert out == Decimal("5")
+
+
+def test_median_percentile_returns_p50() -> None:
+    history = _ramp(200)
+    out = compute_adaptive_threshold(
+        history=history,
+        percentile=Decimal("0.5"),
+        absolute_floor=Decimal("0"),
+        min_days=30,
+        target_days=60,
+    )
+    # P50 di [1..200] = (200+1)/2 = 100.5
+    assert out is not None
+    assert Decimal("100") <= out <= Decimal("101")