feat(combat): wire corner-rule LoS/cover into attack resolution

- weapon_for: gate attacks on has_line_of_effect (no LoE -> policy moves)
- resolve_attack: +4 AC cover bonus on hit and crit-confirm (ranged flag)
- tests: cover bonus (ranged pillar, melee wall corner), no-LoE move/attack,
  no-LoE unreachable wait; 11 resolve_attack call sites updated
- README: quick-start and verified example re-measured (55.7% 1x2), rules
  and architecture updated (los.py wired)

src/pf1e_simulator/combat.py

@@ -9,7 +9,12 @@ Phase 0 documented deviations from PF1e (conventions):
 - Initiative ties: higher initiative_mod first, then list order (no re-roll).
 - Movement: one step per move action toward the nearest enemy, following the
   true shortest path (Dijkstra cost field from the target); ties keep delta order.
-- Ranged attacks ignore cover and range penalties in Phase 0.
+- Line of effect gates all attacks: a target fully behind blocking terrain
+  cannot be attacked, and the policy moves to gain sight instead.
+- Cover (corner rule) grants +4 AC on hit and crit-confirm rolls; melee and
+  ranged reuse the same corner rule.
+- Ranged: only the first range increment is enforced in Phase 0 (no distance
+  penalty, no soft cover from creatures).
 """
 
 from __future__ import annotations
@@ -26,10 +31,13 @@ if TYPE_CHECKING:
     from pf1e_simulator.models import AttackSpec, Combatant, DamageReduction
     from pf1e_simulator.rng import Rng
 
+from pf1e_simulator.los import has_cover, has_line_of_effect
+
 _SQUARE_FT = 5  # Phase 0 maps use 5-ft squares
 _DEATH_FLOOR = -10  # PF1e: dead when hp < -10 or -CON, whichever is lower
 _NATURAL_ONE = 1  # PF1e: natural 1 always misses
 _NATURAL_TWENTY = 20  # PF1e: natural 20 always hits and threatens
+_COVER_AC_BONUS = 4  # PF1e: partial cover grants +4 AC
 
 _STEP_DELTAS: tuple[Pos, ...] = (
     (-1, -1),
@@ -172,7 +180,9 @@ class CombatEngine:
         )
 
     def weapon_for(self, attacker: CombatantState, target: CombatantState) -> AttackSpec | None:
-        """First weapon of the attacker usable against the target at this range."""
+        """First weapon usable against the target at this range and with clear LoE."""
+        if not has_line_of_effect(self._grid, attacker.pos, target.pos):
+            return None
         dist_ft = self._grid.distance(attacker.pos, target.pos) * _SQUARE_FT
         for weapon in attacker.combatant.attacks:
             if weapon.kind in ("melee", "touch") and dist_ft <= weapon.reach_ft:
@@ -186,12 +196,14 @@ class CombatEngine:
         return None
 
     def resolve_attack(
-        self, target: CombatantState, weapon: AttackSpec
+        self, attacker: CombatantState, target: CombatantState, weapon: AttackSpec
     ) -> AttackResult:
         """Roll one attack (with crit confirm and damage) and apply it."""
         roll = self._rng.d20()
         total = roll + weapon.attack_bonus
         ac = target.combatant.ac.total
+        if has_cover(self._grid, attacker.pos, target.pos, ranged=weapon.kind == "ranged"):
+            ac += _COVER_AC_BONUS
         hit = roll == _NATURAL_TWENTY or (roll != _NATURAL_ONE and total >= ac)
         crit = False
         damage = 0
@@ -266,7 +278,7 @@ class CombatEngine:
         self, attacker: CombatantState, target: CombatantState, weapon: AttackSpec, round_no: int
     ) -> None:
         hp_before = target.hp
-        result = self.resolve_attack(target, weapon)
+        result = self.resolve_attack(attacker, target, weapon)
         live = self._stats[attacker.combatant.id]
         live.hits += int(result.hit)
         live.crits += int(result.crit)