Add buffer-native serialization hooks with dynamic write-buffer growth

[Copilot]

This change adds first-class buffer serializers to Storage so callers can bypass string conversion on read/write paths. New serializeToBuffer / deserializeFromBuffer options override the legacy string serializer methods while preserving backward compatibility.

• Serializer API extensions

  • Added config.serializeToBuffer(buffer, document, helpers) => bytesWritten for write-time buffer-native serialization.
  • Added config.deserializeFromBuffer(buffer) => object for read-time buffer-native deserialization.
  • Established precedence: buffer hooks override config.serializer.serialize / deserialize when present.

• Buffer growth contract

  • Introduced helpers.ensureCapacity(minSize) for serializeToBuffer to request a larger working buffer.
  • serializeToBuffer writes into the provided buffer (or expanded one) and returns the exact byte count written.
  • Storage enforces return-value invariants (integer, positive, within buffer bounds).

• Write/read pipeline integration

  • WritableStorage.write() now accepts serializer output as either string or Buffer.
  • WritablePartition write paths (write, buffered/unbuffered writes) now handle Buffer payloads directly.
  • ReadableStorage now deserializes via deserializeFromBuffer when configured, with string fallback unchanged.

• Docs and coverage updates

  • Updated Storage API and advanced docs to describe new options and the growth helper contract.
  • Added tests for override precedence and dynamic buffer growth behavior.

const storage = new Storage('events', {
  writeBufferSize: 64,
  serializeToBuffer: (buffer, doc, { ensureCapacity }) => {
    const encoded = codec.encode(doc); // Buffer
    if (encoded.byteLength > buffer.byteLength) buffer = ensureCapacity(encoded.byteLength);
    encoded.copy(buffer, 0);
    return encoded.byteLength;
  },
  deserializeFromBuffer: (buffer) => codec.decode(buffer)
});