feat: add onUsr1, streq and printOut

user/stderrwriter.c

@@ -0,0 +1,40 @@
+#include "ulib.h"
+
+/*
+ * Live ring-3 proof for shell.c's new `cmd 2> file` / `cmd 2>> file` stderr
+ * redirection (jobctlApplyRedirects/jobctlSpawnForegroundRedirected,
+ * jobctl.h): the exact mirror image of redirwriter.c's own `>`/`>>` proof,
+ * with fd 1 and fd 2's roles swapped -- this program writes its payload to
+ * fd 2 (the fd the shell redirects here) and its own diagnostics to fd 1
+ * (which stays on the real console in this demo, since only fd 2 is ever
+ * redirected). Writing the diagnostics through fd 2 would corrupt the exact
+ * bytes stderrreader.c later compares, the same hazard redirwriter.c avoids
+ * by keeping its own diagnostics off fd 1.
+ *
+ * kmain.c's stderrDriverThread runs this program TWICE against the same
+ * path -- once through `2>` (truncate/create) and once through `2>>`
+ * (append) -- so the resulting file holds two back-to-back copies of this
+ * same payload with no separator, proving `2>>` genuinely continues from
+ * the end of what `2>` already wrote rather than overwriting it.
+ */
+#define stderrPayload "stderr-payload-ok"
+
+static void printOut(const char* text)
+{
+    userWrite(1, text, stringLength(text));
+}
+
+int main(void)
+{
+    printOut("stderrwriter: writing payload to stderr (redirected)\n");
+
+    unsigned long length = stringLength(stderrPayload);
+    long written = userWrite(2, stderrPayload, length);
+    if (written != (long)length) {
+        printOut("stderrwriter: FAIL short/failed write\n");
+        return 1;
+    }
+
+    printOut("stderrwriter: wrote payload\n");
+    return 0;
+}