zero-alloc audio pipeline: pointer-based IO from ring buffer to stdout

Replace Data/AudioPacket allocations with raw pointer callbacks through
the entire audio pipeline. Ring buffer hands out direct pointers (or
linearizes into a pre-allocated scratch buffer on wrap-around), converter
accepts/emits pointers via its cached buffers, and output handler writes
to stdout via write(2) with EINTR handling.

Remove AudioPacket (dead code), --flush flag (no-op with raw write(2)).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Sources/AudioTeeCore/Output/AudioOutputProtocol.swift

@@ -2,7 +2,9 @@ import Foundation
 
 /// Protocol for handling audio output in different formats
 public protocol AudioOutputHandler {
-  func handleAudioPacket(_ packet: AudioPacket)
+  /// Called with a pointer to raw PCM audio data. The pointer is only
+  /// valid for the duration of this call.
+  func handleAudioData(_ pointer: UnsafeRawPointer, count: Int)
   func handleMetadata(_ metadata: AudioStreamMetadata)
   func handleStreamStart()
   func handleStreamStop()