ffi issue-0047: #run print output now routes to stdout

`#run` / post-link callback `print` output was reaching stderr via
`std.debug.print` flushes from three sites. The runtime JIT path
already writes to fd 1 (stdout) directly. Anyone redirecting one
stream saw the two halves disappear in different places.

Switches all three flush sites + the `--- build done ---` delimiter
in main.zig to `std.c.write(1, ...)` so build-time and runtime
prints share the stream the user wrote them against (they typed
the same `print(...)` at both call sites — there's no reason for
them to land on different streams). Test runner uses `2>&1` so
snapshots are unaffected; suite stays at 218/218.

Closes issue-0047.

src/main.zig

@@ -290,7 +290,14 @@ pub fn main(init: std.process.Init) !void {
         // run-time output ambiguously. Only when top-level #run exists —
         // pure-runtime tests keep their current snapshots.
         if (hasTopLevelRun(root)) {
-            std.debug.print("--- build done ---\n", .{});
+            // Stay on the same stream as the #run output (stdout, via
+            // core.flushInterpOutput). Same reason as issue-0047: the
+            // user doesn't distinguish build-time `print` from
+            // runtime `print` at the call site, and the delimiter is
+            // meaningless if it lands on a different stream than the
+            // output it's separating.
+            const marker = "--- build done ---\n";
+            _ = std.c.write(1, marker.ptr, marker.len);
         }
         const exit_code = sx.target.runJITFromObject(obj_buf) catch {
             // JIT failed — fall back to AOT