Complete the switch to jnk0le uart lib

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/avclan/hal/stdio.h

@@ -0,0 +1,20 @@
+// Copyright (C) 2026 Allen Hill <allenofthehills@gmail.com>
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+#pragma once
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+// Generic stdio interface initialization. All user I/O goes through <stdio.h>
+// functions. Assumptions/invariants:
+//  - stdin MUST be non-blocking (ie. getchar() returns EOF immediately when
+//    empty). Necessary to avoid stalling the REPL poll loop.
+//  - stdout is *raw* — no '\n' -> "\r\n" translation: bare LF and binary frame
+//    payloads (fwrite) are emitted verbatim
+void stdio_init(void);
+
+#ifdef __cplusplus
+}
+#endif