selfhost: port switch — N_SWITCH parser + cgen + scratch slot

selfhost/cmd/wcc/cgendecl.ww

@@ -87,6 +87,11 @@ fn scanlocals(c: *cgen, n: *node) i32 = {
 			bidx += 1;
 		};
 	};
+	// `switch` allocates an 8B scratch slot for the scrutinee so case
+	// bodies can spill through SP without losing it. The slot is named
+	// ".sw_<labelseq>" at cgen time — unique per switch — so it must
+	// not dedup. Count it here so the frame SUBQ matches.
+	if (n.kind == nkind.N_SWITCH) { total += 8; };
 	// Match-arm binding (`case let v: T => ...`) gets a slot too.
 	// Crucially we do NOT dedup these against c.locals: C cgen
 	// handles a match as an expression with a by-value locals copy,