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

selfhost/cmd/wcc/cgen.ww

@@ -524,6 +524,37 @@ fn emitlabel(s: str) void = {
 	emitline(":\n");
 };
 
+// mkscratchname — fresh local-slot name ".<base>_<labelseq>". Used for
+// compiler-synthesised slots (switch scrutinee, forrange index/len)
+// that need to be unique per use site but are never referenced by user
+// code. Increments labelseq so the same source position lines up with
+// C cgen's labelseq stream.
+fn mkscratchname(c: *cgen, base: str) str = {
+	let buf: [128]u8;
+	let i: i32 = 0;
+	buf[i] = 46u8; i += 1;	// '.'
+	let j: i32 = 0;
+	for (j < base.len) {
+		buf[i] = base[j];
+		i += 1; j += 1;
+	};
+	buf[i] = 95u8; i += 1;	// '_'
+	let n: i32 = strconv.i64tos(buf[i:128], c.labelseq: i64);
+	c.labelseq += 1;
+	let total: i32 = i + n;
+	let p: *u8 = amalloc(c.a, (total: u64) + 1u64): *u8;
+	let k: i32 = 0;
+	for (k < total) {
+		p[k] = buf[k];
+		k += 1;
+	};
+	p[total] = 0u8;
+	let r: str;
+	r.ptr = p;
+	r.len = total;
+	return r;
+};
+
 // ---- string interning ------------------------------------------------
 //
 // streq is provided by sym.ww and reused here.