wcc: tuple slice-element read loads the full 24B header, both stages

Reading a slice-typed tuple element (t.0) loaded only the pointer
word; len and cap took whatever was left in BX/CX — silent garbage in
BOTH stages once anything clobbered the registers between build and
read. Load all three header words at the tuple-element arm. Review
item #28.

Both stages move in one commit: one emission contract; splitting
would leave the byte-id gates red between the halves.

test/wcc/949_f6_header_run.c

@@ -100,6 +100,45 @@ static const struct row rows[] = {
 	  "	let v: []u8 = s: []u8;\n"
 	  "	return v.cap: i32;\n"
 	  "};\n", 4, K_RUN, NULL },
+	/* #28 (#263 both-stages): tuple slice-element read `t.0` over a
+	 * ([]u8, i64). A clobber() call between the tuple build and the
+	 * read leaves junk in BX/CX; pre-fix the str-only arm fell to the
+	 * scalar tail (ptr word only), so ys.len read stale BX. Reads len
+	 * == 5 (distinct from any stale value) → 0. */
+	{ "tuple_slice_elem",
+	  "package main;\n"
+	  "fn clobber() i64 = {\n"
+	  "	let a: i64 = 111;\n"
+	  "	let b: i64 = 222;\n"
+	  "	return a + b;\n"
+	  "};\n"
+	  "export fn main() i32 = {\n"
+	  "	let xs: []u8 = [1u8, 2u8, 3u8, 4u8, 5u8];\n"
+	  "	let t: ([]u8, i64) = (xs, 7);\n"
+	  "	let junk: i64 = clobber();\n"
+	  "	let ys: []u8 = t.0;\n"
+	  "	if (ys.len == 5) { return 0; };\n"
+	  "	return 1;\n"
+	  "};\n", 0, K_RUN, NULL },
+	/* #28 sibling: the slice element sits at tuple position 1 (foff != 0:
+	 * an i64 occupies slot 0, the []u8 header starts at +8). Exercises the
+	 * widened arm's offset arithmetic, which the position-0 row leaves
+	 * untested. Same clobber()-between-build-and-read trigger; len == 6. */
+	{ "tuple_slice_elem_pos1",
+	  "package main;\n"
+	  "fn clobber() i64 = {\n"
+	  "	let a: i64 = 111;\n"
+	  "	let b: i64 = 222;\n"
+	  "	return a + b;\n"
+	  "};\n"
+	  "export fn main() i32 = {\n"
+	  "	let xs: []u8 = [1u8, 2u8, 3u8, 4u8, 5u8, 6u8];\n"
+	  "	let t: (i64, []u8) = (9, xs);\n"
+	  "	let junk: i64 = clobber();\n"
+	  "	let ys: []u8 = t.1;\n"
+	  "	if (ys.len == 6) { return 0; };\n"
+	  "	return 1;\n"
+	  "};\n", 0, K_RUN, NULL },
 };
 
 static int