feat(combat): support partial and improved cover levels

src/pf1e_simulator/map.py

@@ -11,7 +11,7 @@ from __future__ import annotations
 from typing import TYPE_CHECKING
 
 import yaml
-from pydantic import BaseModel, ConfigDict, Field, ValidationError
+from pydantic import BaseModel, ConfigDict, Field, ValidationError, field_validator
 
 if TYPE_CHECKING:
     from pathlib import Path
@@ -20,6 +20,8 @@ Pos = tuple[int, int]  # (row, col), 0-indexed
 
 _ELEVATION_CHARS = frozenset("0123456789.")  # digit = height, '.' = ground level
 
+_COVER_BONUS_LEVELS = frozenset({0, 2, 4, 8})  # PF1e: none / partial / standard / improved
+
 
 class MapValidationError(Exception):
     """Raised when a map YAML file fails validation; carries path and reason."""
@@ -38,8 +40,17 @@ class TerrainType(BaseModel):
     type: str
     move_cost: int | None = Field(default=None, ge=1)
     cover: bool = False
+    cover_bonus: int = 0
     blocks_los: bool = False
 
+    @field_validator("cover_bonus", mode="after")
+    @classmethod
+    def _check_cover_bonus(cls, value: int) -> int:
+        if value not in _COVER_BONUS_LEVELS:
+            msg = f"cover_bonus must be one of {sorted(_COVER_BONUS_LEVELS)}, got {value}"
+            raise ValueError(msg)
+        return value
+
 
 class MapSpec(BaseModel):
     """A fully validated map, ready for Grid construction."""