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

src/pf1e_simulator/combat.py

@@ -769,9 +769,13 @@ class CombatantState:
     @property
     def death_threshold(self) -> int:
         """PF1e (CRB): death when negative hp reaches the CON score (no -10 rule).
-        The optional simple monster-death rule raises the threshold to 0."""
+        Undead and constructs have no Constitution score and are "immediately
+        destroyed when reduced to 0 hit points or fewer" (SRD Undead/Construct
+        traits). The optional simple monster-death rule raises the threshold to 0."""
         if self.dies_at_zero:
             return 0
+        if self.combatant.creature_type in ("undead", "construct"):
+            return 0
         return -self.combatant.abilities.con_score