Complete the switch to jnk0le uart lib

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

src/avclan/target/avr-attiny3216/stdio_avr.c

@@ -0,0 +1,47 @@
+// Copyright (C) 2026 Allen Hill <allenofthehills@gmail.com>
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+#include <avr/io.h>
+#include <stdint.h>
+#include <stdio.h>
+
+#include "hal/stdio.h"
+#include "timing_avr.h" // IWYU pragma: export
+#include "usart.h"      // jnk0le AVR-UART-lib
+
+// Raw stdout backend: emit the byte verbatim, no '\n' -> "\r\n" translation
+// (unlike the lib's uart_putchar). Keeps binary frame payloads intact.
+static int stdio_putchar(char data, FILE *stream) {
+  (void)stream;
+  uart0_putc(data);
+  return 0;
+}
+
+// Non-blocking stdin backend: next received byte, or _FDEV_EOF when the RX ring
+// is empty (uart0_getData() returns a negative value when there is no data).
+static int stdio_getchar(FILE *stream) {
+  (void)stream;
+  int16_t c = uart0_getData();
+  return c < 0 ? _FDEV_EOF : c;
+}
+
+// One raw RW stream for the stdio UART: verbatim byte output, non-blocking
+// input.
+static FILE stdio_stream =
+    FDEV_SETUP_STREAM(stdio_putchar, stdio_getchar, _FDEV_SETUP_RW);
+
+void stdio_init(void) {
+  // The lib's uart0_init configures the USART registers/baud but not the pins;
+  // keep the ATtiny3216 pin-mux the old driver did.
+  PORTMUX.CTRLB = PORTMUX_USART0_ALTERNATE_gc; // TxD/RxD on PA1/PA2
+  PORTA.DIRSET = PIN1_bm;                       // TxD output
+  PORTA.DIRCLR = PIN2_bm;                       // RxD input
+
+  #ifdef USE_DOUBLE_SPEED
+  uart0_init(DOUBLE_BAUD_CALC(1200000));
+  #else
+  uart0_init(BAUD_CALC(1200000));
+  #endif
+
+  stdout = stdin = &stdio_stream; // printf/fputs/fwrite + non-blocking getchar
+}