optimise hot-path audio pipeline: zero-copy ring buffer, pre-allocated converter buffers

- Replace Swift Array<UInt8> ring buffer with UnsafeMutableRawPointer to
  eliminate COW ref-count checks on every write/read
- Add append(from:count:) to copy directly from Core Audio buffer pointer
  into the ring buffer, removing the per-callback Data heap allocation
- Pre-allocate AVAudioPCMBuffer pair in AudioFormatConverter and reuse
  across transform() calls (lazy init, capacity-checked)
- Fix float-to-int truncation in output frame count calculation (ceil)
- Add comprehensive AudioBuffer test suite (12 tests) including proper
  wrap-around coverage for both append and read paths

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

Tests/AudioTeeCoreTests/AudioPacketTests.swift

@@ -1,4 +1,5 @@
 import XCTest
+
 @testable import AudioTeeCore
 
 final class AudioPacketTests: XCTestCase {
@@ -6,25 +7,25 @@ final class AudioPacketTests: XCTestCase {
     let timestamp = Date()
     let duration = 1.0
     let data = Data([0x01, 0x02, 0x03, 0x04])
-    
+
     let packet = AudioPacket(
       timestamp: timestamp,
       duration: duration,
       data: data
     )
-    
+
     XCTAssertEqual(packet.timestamp, timestamp)
     XCTAssertEqual(packet.duration, duration)
     XCTAssertEqual(packet.data, data)
   }
-  
+
   func testPacketDataSize() {
     let packet = AudioPacket(
       timestamp: Date(),
       duration: 0.5,
       data: Data(repeating: 0xFF, count: 1024)
     )
-    
+
     XCTAssertEqual(packet.data.count, 1024)
   }
-}
+}