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.

tests/expected/239-main-signature-reject.txt

@@ -1,5 +1,5 @@
 error: main: return type must be void, an integer, or `!`; got 'string'
-  --> /Users/agra/projects/sx/examples/239-main-signature-reject.sx:15:15
+  --> /Users/agra/projects/sx/examples/239-main-signature-reject.sx:13:15
    |
-15 | main :: () -> string {        // ERROR: return type must be void, an integer, or `!`
+13 | main :: () -> string {        // ERROR: return type must be void, an integer, or `!`
    |               ^^^^^^

tests/expected/245-failable-main-value.exit

@@ -0,0 +1 @@
+64

tests/expected/245-failable-main-value.txt

@@ -0,0 +1 @@
+v = 64