refactor(protocol): extract _READ_CHUNK_SIZE constant

Replace the bare literal 4096 in DqliteProtocol._read_chunk with a
named module-level constant and explain the tradeoff (syscall overhead
vs. small-message latency).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: antoineleclair

src/dqliteclient/protocol.py

@@ -27,6 +27,10 @@
 )
 from dqlitewire.messages.base import Message
 
+# Socket read buffer size. 4 KiB balances syscall overhead for typical
+# request/response payloads against latency for small wire messages.
+_READ_CHUNK_SIZE = 4096
+
 
 class DqliteProtocol:
     """Low-level protocol handler for a single dqlite connection."""
@@ -292,7 +296,7 @@ async def _read_data(self, deadline: float | None = None) -> bytes:
         else:
             timeout = self._timeout
         try:
-            data = await asyncio.wait_for(self._reader.read(4096), timeout=timeout)
+            data = await asyncio.wait_for(self._reader.read(_READ_CHUNK_SIZE), timeout=timeout)
         except TimeoutError:
             raise DqliteConnectionError(f"Server read timed out after {timeout:.1f}s") from None
         except (ConnectionError, OSError, RuntimeError) as e: