fix(map): fail on blank rows inside a layer, keep trailing-space cells verbatim

src/pf1e_simulator/map.py

@@ -89,14 +89,27 @@ class _MapFile(BaseModel):
 
 
 def _split_rows(block: str, what: str, path: Path) -> tuple[str, ...]:
-    rows = tuple(line.rstrip() for line in block.splitlines() if line.strip())
-    if not rows:
+    """Split an ASCII layer into equal-width rows, failing loudly on gaps.
+
+    Trailing empty lines are a YAML block-scalar artifact and dropped; an
+    empty line *inside* the layer is a missing row -- a data error, never a
+    silently dropped one. Rows are kept verbatim (no rstrip): trailing spaces
+    are cells whenever " " is in the legend, and the width/char checks catch
+    them otherwise.
+    """
+    lines = block.splitlines()
+    while lines and lines[-1] == "":
+        lines.pop()
+    if not lines:
         msg = f"{what} layer is empty"
         raise MapValidationError(path, msg)
-    if len({len(row) for row in rows}) != 1:
+    if any(line == "" for line in lines):
+        msg = f"{what} layer has a blank row inside the layer"
+        raise MapValidationError(path, msg)
+    if len({len(row) for row in lines}) != 1:
         msg = f"{what} layer has ragged rows"
         raise MapValidationError(path, msg)
-    return rows
+    return tuple(lines)
 
 
 def _check_layer_dimensions(