fix(rules): undead and constructs are destroyed at hp <= 0

SRD Undead/Construct traits: 'immediately destroyed when reduced to 0 hit
points or fewer' — undead and constructs have no Constitution score, so
the PF1e hp <= -CON death rule does not apply to them.

- combat.py: CombatantState.death_threshold returns 0 for creature_type
  undead/construct (in addition to the opt-in dies_at_zero flag)
- tests: TestUndeadDeathThreshold pins undead dead at 0 and below, living
  keeps the disabled band at 0 and the dying band down to -CON
- golden unchanged: tammara-vs-lich has the lich winning in 1 round
  without taking damage

tests/test_lich.py

@@ -34,6 +34,7 @@ def _make_lich() -> Combatant:
         name="Lich",
         level=11,
         size="Medium",
+        creature_type="undead",
         abilities=AbilityScores(
             str_score=12, dex_score=14, con_score=14,
             int_score=20, wis_score=14, cha_score=16,
@@ -173,6 +174,42 @@ class TestFearAura:
         assert all(ac.condition.name != "shaken" for ac in foe_state.active_conditions)
 
 
+# ── Undead death threshold ────────────────────────────────────────────────────
+
+
+class TestUndeadDeathThreshold:
+    """Given: an undead creature (no CON score, SRD Undead traits)
+    When:  its hp reaches 0 or below
+    Then:  it is immediately destroyed (dead), unlike a living creature
+    which keeps the disabled band at 0 and the dying band down to -CON."""
+
+    def test_undead_dead_at_zero(self) -> None:
+        lich = _make_lich()
+        state = _make_state(lich, "monsters", (1, 1))
+        state.hp = 0
+        assert state.dead is True
+        assert state.active is False
+
+    def test_undead_dead_below_zero(self) -> None:
+        lich = _make_lich()
+        state = _make_state(lich, "monsters", (1, 1))
+        state.hp = -1
+        assert state.dead is True
+
+    def test_living_not_dead_at_zero(self) -> None:
+        foe = _make_foe()
+        state = _make_state(foe, "players", (1, 1))
+        state.hp = 0
+        assert state.dead is False
+        assert state.active is True
+
+    def test_living_not_dead_in_dying_band(self) -> None:
+        foe = _make_foe()
+        state = _make_state(foe, "players", (1, 1))
+        state.hp = -1  # -1 > -CON (12)
+        assert state.dead is False
+
+
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