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).

src/target.zig

@@ -235,10 +235,14 @@ pub fn runJITFromObject(obj_buf: c.LLVMMemoryBufferRef) !u8 {
         return error.CompileError;
     }
 
-    // Cast to function pointer and call
+    // Cast to function pointer and call. The exit code is main's integer
+    // return truncated to u8 — matching the OS truncation an AOT binary's
+    // exit status already gets, so JIT and AOT agree (e.g. 1105 -> 81,
+    // -1 -> 255, 256 -> 0). Bit-cast i32 -> u32 first so negatives wrap
+    // as two's-complement low byte rather than being clamped.
     const main_fn: *const fn () callconv(.c) i32 = @ptrFromInt(main_addr);
     const result = main_fn();
-    return if (result >= 0 and result <= 255) @intCast(result) else 1;
+    return @truncate(@as(u32, @bitCast(result)));
 }
 
 // Android APK bundling (createApk, compileJniMainSources,