feat(codec): negotiate checksum/compression algorithms (protocol 2.26.0)

Accept the full rsync 3.4.1 --compress-choice set (zstd/lz4/zlib/zlibx/
none/auto) and the two-name --checksum-choice TRANSFER,PRE-TRANSFER form,
including rsync's 'none' rules (rejected with --checksum at exit 4, and
forcing --whole-file as the transfer half) and unknown names at exit 4.
The checksum default becomes the auto-negotiated xxh128.

Negotiation is deterministic and symmetric: both peers run the same
preference resolver (rsync's --version order).  The resolved
compression_algo crosses the wire as a new trailing config-frame int so
the receiver validates and installs the exact codec; an unsupported
choice is refused before STATUS_OK like rsync's failed negotiation.
Bump PROTOCOL_VERSION to 2.26.0.

src/server/server.c

@@ -741,6 +741,10 @@ void handler(int file_descriptor) {
    * received config. */
   if (gate_ctx.super_mode_override != -1)
     config->super_mode = (SuperMode)gate_ctx.super_mode_override;
+  /* Install the codec this connection negotiated before the receiver/writer
+   * threads start (the server forks per connection, so the process-global
+   * codec is private to this session). */
+  compression_set_algo((CompressionAlgo)config->compression_algo);
   /* If the client requested ownership but the effective super mode forbids it
    * (operator --no-super, a privileged standalone receiver's secure default, or
    * a daemon module without `client owner = yes`), say so ONCE per connection so