ERR/E4.2: truncate integer main's return to u8 for the JIT exit code

A non-failable integer `main :: () -> T` must exit with its return value
truncated to u8 (matching C main / the OS exit-status byte), so `sx run`
(JIT) and an AOT binary agree. runJITMain clamped instead: any value outside
0..255 returned exit 1, so `return 1105` exited 1 (not 81), `return -1` exited
1 (not 255), and `return 256` exited 1 (not 0).

Fix: bit-cast the i32 return to u32 and @truncate to u8 — negatives wrap as
their two's-complement low byte rather than being clamped. The AOT path
already gets OS truncation, so it was already correct; this makes JIT match.

examples/238-main-exit-truncation.sx returns 1105 -> exit 81. Values <=255
(42, 200) still pass through unchanged.

Gates: zig build, zig build test, bash tests/run_examples.sh (275 passed; the
lone failure is the user's uncommitted 213-canonical-map pack WIP).

examples/238-main-exit-truncation.sx

@@ -0,0 +1,14 @@
+// Entry-point exit-code truncation (ERR step E4.2, non-failable integer main).
+// `main :: () -> T` (integer) exits with the return value truncated to u8 —
+// matching C `main` / the OS exit-status byte, so the JIT (`sx run`) and an AOT
+// binary agree. Here 1105 & 0xFF == 81, so this program exits 81 (NOT 1, which
+// was the old buggy "out of 0..255 range -> failure" behavior).
+//
+// Run: ./zig-out/bin/sx run examples/238-main-exit-truncation.sx ; echo $?   # 81
+
+#import "modules/std.sx";
+
+main :: () -> s32 {
+    print("returning 1105 -> exit {}\n", 1105 & 0xFF);   // 81
+    return 1105;
+}