ERR/E4.2: value-carrying -> (int, !) main wrapper

Extends the failable-main entry-point wrapper to a value-carrying main.
`main :: () -> (int, !)` now exits the integer value on success (truncated
to u8, like a plain integer main) and reports the header + trace to stderr
+ exits 1 on an escaping error (same reporter as the pure `-> !` form).

- lower.zig validateMainSignature: accept a 2-field `{int, error_set}`
  tuple return (set needs_trace_runtime) instead of rejecting it. Multi-
  value `-> (T1, T2, !)` and non-integer value slots still reject — there's
  no single integer exit code to map them to (sharpened diagnostic).
- emit_llvm.zig: the `.ret` arm detects a value-carrying main (tuple ending
  in `.error_set`) and extracts `{value, tag}` (extractvalue 0/1) before
  calling emitFailableMainRet, now generalized to take an optional `value`
  (null → pure `-> !`, success exits 0; present → success exits the value).
  C reporter unchanged.

All E4.2 entry-point shapes (void / int / `-> !` / `-> (int, !)`) now done.
examples/245-failable-main-value.sx (exit 64); 239 comment refreshed.

examples/245-failable-main-value.sx

@@ -0,0 +1,22 @@
+// Value-carrying failable main `-> (int, !)` (ERR step E4.2). The entry-point
+// wrapper extracts the `{value, error}` tuple main returns: on success it exits
+// with the integer value (truncated to u8, like a plain integer main); on an
+// escaping error it prints the header + trace to stderr and exits 1 (the same
+// reporter as the pure `-> !` form — see 244). This run takes the success path.
+// Expected exit code: 64 (the returned value).
+
+#import "modules/std.sx";
+
+ParseErr :: error { Empty, BadDigit };
+
+inner :: (n: s32) -> (s32, !ParseErr) {
+    if n == 0 { raise error.Empty; }
+    if n < 0 { raise error.BadDigit; }
+    return n * 2;
+}
+
+main :: () -> (s32, !ParseErr) {
+    v := try inner(32);                      // succeeds → v = 64
+    print("v = {}\n", v);
+    return v;                                // success → exit code 64
+}