feat(uds): add Store opcode, AppendRawNeedle, and SRA transport client

Volume replication groundwork (Phase 6 hardening plan):

- UDS handler: Add Store opcode (0x02) with opcode-based dispatch,
  StoreRequest/StoreResponse wire types, and size validation
- storage/store.go: AppendRawNeedle() for accepting pre-serialized
  needle bytes from RDMA replication path
- storage/needle: Return raw bytes from needle Append() to enable
  forwarding exact .dat bytes without re-serialization
- storage/sra_transport.go: UDS client for outbound RDMA replication
  (connects to /tmp/sra-transport.sock, nil-safe fallback to HTTP)
- command/volume.go: Wire -uds.transport flag
- Integration test for Store opcode round-trip
- Golden wire tests for Store request/response encoding

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

weed/command/server.go

@@ -153,6 +153,7 @@ func init() {
 	serverOptions.v.inflightDownloadDataTimeout = cmdServer.Flag.Duration("volume.inflightDownloadDataTimeout", 60*time.Second, "inflight download data wait timeout of volume servers")
 
 	serverOptions.v.udsListen = cmdServer.Flag.String("volume.uds.listen", "", "Unix domain socket path for RDMA sidecar locate API (e.g., /tmp/sra-volume.sock)")
+	serverOptions.v.udsTransport = cmdServer.Flag.String("volume.uds.transport", "", "Unix domain socket path for outbound RDMA replication (e.g., /tmp/sra-transport.sock)")
 
 	serverOptions.v.hasSlowRead = cmdServer.Flag.Bool("volume.hasSlowRead", true, "<experimental> if true, this prevents slow reads from blocking other requests, but large file read P99 latency will increase.")
 	serverOptions.v.readBufferSizeMB = cmdServer.Flag.Int("volume.readBufferSizeMB", 4, "<experimental> larger values can optimize query performance but will increase some memory usage,Use with hasSlowRead normally")