feat(combat): expose threatened squares and movement cost fields for LLM policy

Rename _threatened_squares -> threatened_squares and
_enemies_threatening -> enemies_threatening (public read-only API for a
future LLM policy), and add reachable_ft(state, budget) and
movement_cost_ft(state, target) accessors returning movement costs in
feet. Pure exposure: no behavior change, resolution engine untouched,
goldens bit-identical.

tests/test_creature_sizes.py

@@ -256,7 +256,7 @@ class TestThreatenedSquaresLarge:
         giant = make_combatant("g", size="Large")
         state = make_state(giant, "p", (2, 2))
         engine = make_engine([state])
-        threatened = engine._threatened_squares(state)
+        threatened = engine.threatened_squares(state)
         # Must contain cells adjacent to any of the 4 occupied cells
         # e.g. (0,2) is at distance 2 from (2,2), should be in
         assert (0, 2) in threatened
@@ -287,7 +287,7 @@ class TestThreatenedSquaresLarge:
         )
         state = make_state(combatant, "p", (2, 2))
         engine = make_engine([state])
-        threatened = engine._threatened_squares(state)
+        threatened = engine.threatened_squares(state)
         # Ring at distance 2 from each occupied cell is included
         # (2,2) ring(2) includes (0,2) etc.
         # (3,3) ring(2) includes (1,3), (5,3), (3,1), (3,5) etc.