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

test/wcc/990_selfhost.c

@@ -345,6 +345,21 @@ probe_ww_compile(const char *bin)
 		  "  dec(&c); dec(&c); dec(&c);\n"
 		  "  return c.x;\n"
 		  "};", 2 },
+		/* switch with multi-expr cases + default. Lowers to a chain
+		 * of CMPQ + JE; the scrutinee lands in a fresh 8B local slot
+		 * (".sw_N") so case bodies can spill SP without losing it.
+		 * classify(2)=10, classify(7)=70, classify(99)=0, sum 80. */
+		{ "fn classify(x: i32) i32 = {\n"
+		  "  switch (x) {\n"
+		  "    case 1, 2, 3: return 10;\n"
+		  "    case 7: return 70;\n"
+		  "    case: return 0;\n"
+		  "  };\n"
+		  "  return -1;\n"
+		  "};\n"
+		  "fn main() i32 = {\n"
+		  "  return classify(2) + classify(7) + classify(99);\n"
+		  "};", 80 },
 		/* Chained N_DOT: o.ptr_to_inner.val (read field through a
 		 * pointer-to-struct field). Pre-fix the cgen fell through
 		 * silently and returned the inner pointer instead of the