population: extract strategy/population.py; live gate delegates to it

The tradeable population is now defined ONCE. trading_execution._population_reject
becomes a thin wrapper building a PopulationConfig from its settings and calling
strategy.population.population_reject — logic moved verbatim (dead-band 0.35/1.50
and squeeze rvol 5.0 are module CONSTANTS, gap floor / price band / volume floor
from config). Behaviour byte-identical: the pre-extraction characterization suite
stays green through the new public seam.

from_env mirrors ExecutionSettings.from_env (same 4 keys + the [0,1] _qfloat clamp)
so a live-shaped shell reconstructs the identical gate. gap_pct FRACTION contract
and the as_of open-knowable contract are documented for offline consumers.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/unit/strategy/test_population_module.py

@@ -0,0 +1,62 @@
+"""Module-level tests for strategy/population.py — the shared population gate.
+
+Complements tests/unit/test_population_gate.py (which pins the LIVE seam through
+the service). Here we test the module directly: from_env key parity with
+production, the as_of contract docstring, and fail-open semantics.
+"""
+
+import inspect
+
+from strategy.population import PopulationConfig, population_reject
+
+
+def test_from_env_defaults_match_production_keys():
+    cfg = PopulationConfig.from_env(env={
+        "TRADING_PRICE_MIN": "2.0",
+        "TRADING_PRICE_MAX": "20.0",
+        "TRADING_MIN_ENTRY_GAP_PCT": "0.10",
+        "TRADING_MIN_DAY_VOLUME": "300000",
+    })
+    assert cfg.price_min == 2.0
+    assert cfg.price_max == 20.0
+    assert cfg.min_entry_gap_pct == 0.10
+    assert cfg.min_day_volume == 300_000
+    # constants are NOT env-driven
+    assert cfg.gap_dead_band_lo == 0.35
+    assert cfg.gap_dead_band_hi == 1.50
+    assert cfg.squeeze_rvol_min == 5.0
+
+
+def test_from_env_bare_shell_disables_gap_gate():
+    # No production env => min_entry_gap_pct clamps to 0.0 (gap gate off), the
+    # exact hazard the plan warns offline consumers about.
+    cfg = PopulationConfig.from_env(env={})
+    assert cfg.min_entry_gap_pct == 0.0
+    assert population_reject(cfg, gap_pct=0.05, price=10.0, volume=None, rvol=None) is None
+
+
+def test_from_env_clamps_gap_floor_to_unit_interval():
+    # mirrors trading_execution._qfloat: percent-typed 40 must NOT survive as 40.
+    assert PopulationConfig.from_env(env={"TRADING_MIN_ENTRY_GAP_PCT": "40"}).min_entry_gap_pct == 1.0
+    assert PopulationConfig.from_env(env={"TRADING_MIN_ENTRY_GAP_PCT": "junk"}).min_entry_gap_pct == 0.0
+
+
+def test_as_of_contract_documented():
+    # day-end rejection is by-convention (review-enforced), so the contract MUST
+    # be spelled out in the signature + docstring.
+    assert "as_of" in inspect.signature(population_reject).parameters
+    doc = population_reject.__doc__ or ""
+    assert "FRACTION" in doc and "forbidden" in doc.lower()
+
+
+def test_fail_open_on_all_missing():
+    cfg = PopulationConfig(min_entry_gap_pct=0.10, min_day_volume=300_000)
+    assert population_reject(cfg, gap_pct=None, price=None, volume=None, rvol=None) is None
+
+
+def test_offline_consumer_must_pass_prod_thresholds():
+    # bare config admits everything; explicit prod thresholds gate correctly.
+    bare = PopulationConfig()
+    assert population_reject(bare, gap_pct=0.02, price=3.0, volume=1, rvol=None) is None
+    prod = PopulationConfig(min_entry_gap_pct=0.10, min_day_volume=300_000)
+    assert population_reject(prod, gap_pct=0.02, price=3.0, volume=1, rvol=None) is not None