Speed up Frame printing to (finally) solve missing AVCLAN bus frames

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

src/avclan/hal/stdio.h

@@ -3,6 +3,8 @@
 
 #pragma once
 
+#include <stddef.h>
+
 #ifdef __cplusplus
 extern "C" {
 #endif
@@ -11,10 +13,25 @@ extern "C" {
 // 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
+//  - stdout is *raw*. There is no '\n' -> "\r\n" translation. The port does not
+//    change a bare LF or a binary frame payload. Writes through
+//    <stdio.h> and writes through stdio_write_nonblock() must reach the same
+//    stream, in order.
 void stdio_init(void);
 
+// Queue a buffer onto stdout--if it has space, or return false.
+//
+// If stdout cannot accept all `len` bytes now, the function queues
+// a "\n!\n" indicator onto stdout, overwriting the last 3 chars if stdout is
+// full. The caller must not assume that the buffer went out.
+//
+// A port MUST emit that indicator for every dropped buffer, and MUST be able to
+// emit it even if stdout is full. A reader detects the loss as a line that
+// holds only '!'. In the overwrite case the buffer in front of that line is
+// truncated, so a reader MUST tolerate one malformed buffer there. The return
+// value is useful only for diagnostics.
+bool stdio_write_nonblock(const void *buf, size_t len);
+
 #ifdef __cplusplus
 }
 #endif