Mask the node state out of the operation when marking shared nodes

Geometry that was clipped to --clip-bounding-box while it was being
serialized can already have node states in its operation bytes, so the
unmasked comparison against VT_MOVETO and VT_LINETO failed to recognize
those vertices and lost track of where the following vertices began.
Also stop with an error instead of continuing if the operation is not
one that can appear in serialized geometry.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01P2nBqZisxNQfmEmon3vE9v

serial.cpp

@@ -345,6 +345,9 @@ void mark_shared_nodes(char *feature, size_t len, unsigned *initial_x, unsigned
 			return;
 		}
 
+		// the vertex may already have a node state, from clipping to --clip-bounding-box
+		op &= OP_MASK;
+
 		if (op == VT_MOVETO || op == VT_LINETO) {
 			long long dx, dy;
 
@@ -356,6 +359,9 @@ void mark_shared_nodes(char *feature, size_t len, unsigned *initial_x, unsigned
 
 			signed char node = is_shared_node(wx, wy, shared_nodes_map, nodepos, shared_nodes_bloom) ? NODE_SHARED : NODE_NOT_SHARED;
 			*opp = op | (node << NODE_SHIFT);
+		} else if (op != VT_CLOSEPATH) {
+			fprintf(stderr, "Internal error: unexpected geometry operation %d marking shared nodes\n", op);
+			exit(EXIT_IMPOSSIBLE);
 		}
 	}