fix: add creature damage accumulation to combat engine

Previously _resolve_combat() used a binary power-vs-toughness kill check:
creatures only died if a single source had power >= their toughness in one
swing. This meant a 2/2 wouldn't die to two 1/1 tokens blocking it, and
repeated smaller hits across combats never accumulated.

Changes:
- models.py: add damage_marked field to Card (default 0)
- engine.py: accumulate damage via damage_marked += power instead of
  binary a_pow >= toughness checks in _resolve_combat()
- engine.py: clear damage_marked = 0 in untap phase (MTG rule 514.2)

This enables multi-blocker lethality, incremental chip damage within a
combat step, and proper creature death-by-accumulated-damage interactions.

src/commander_ai_lab/sim/engine.py

@@ -127,6 +127,7 @@ class GameEngine:
         # ── Untap (correct MTG order: untap is first) ──
         for c in sim.get_battlefield(pi):
           c.tapped = False
+          c.damage_marked = 0  # clear damage each turn (MTG rule 514.2)
 
         # ── Draw ──
         if p.library:
@@ -590,12 +591,15 @@ class GameEngine:
           blocked = True
           if events is not None:
             combat_details.append(f"{atk.name} blocked by {blocker.name}")
-          if a_pow >= blocker.get_toughness() or atk.has_keyword("deathtouch"):
+          # Accumulate damage on creatures (not binary kill)
+          blocker.damage_marked += a_pow
+          if blocker.damage_marked >= blocker.get_toughness() or atk.has_keyword("deathtouch"):
             sim.remove_from_battlefield(blocker.id)
             self._send_to_graveyard(sim, blocker, opp_idx)
             if events is not None:
               combat_details.append(f"  {blocker.name} dies")
-          if b_pow >= a_tou or blocker.has_keyword("deathtouch"):
+          atk.damage_marked += b_pow
+          if atk.damage_marked >= a_tou or blocker.has_keyword("deathtouch"):
             sim.remove_from_battlefield(atk.id)
             self._send_to_graveyard(sim, atk, pi)
             if events is not None:

src/commander_ai_lab/sim/models.py

@@ -43,6 +43,7 @@ class Card:
     owner_id: int = -1
     tapped: bool = False
     turn_played: int = -1
+    damage_marked: int = 0  # track damage marked on creature (cleared each cleanup step)
 
     def get_power(self) -> int:
         """Extract numeric power from pt string."""