Stream slave accel via ESP-NOW with master snapshot cache.

Slaves push BMA456 samples at 16ms when enabled; the master caches per
client and exposes ACCEL_SNAPSHOT and ACCEL_STREAM over UART. goTool adds
dashboard stream controls, HTTP accel-stream routes, and an external
WebSocket API with per-connection receive/interval and slave stream commands.

Co-authored-by: Cursor <cursoragent@cursor.com>

goTool/main.go

@@ -16,7 +16,7 @@ func usage() {
 	fmt.Fprintf(os.Stderr, "  version   firmware version and git hash\n")
 	fmt.Fprintf(os.Stderr, "  clients   registered ESP-NOW slaves on the master\n")
 	fmt.Fprintf(os.Stderr, "  deadzone      get/set accelerometer deadzone (LSB)\n")
-	fmt.Fprintf(os.Stderr, "  accel         read current accelerometer XYZ (raw LSB)\n")
+	fmt.Fprintf(os.Stderr, "  accel         read cached slave accel snapshot from master\n")
 	fmt.Fprintf(os.Stderr, "  unicast-test  send ESP-NOW unicast test to one slave\n")
 	fmt.Fprintf(os.Stderr, "  test          run automated scenario (see testdata/)\n")
 	fmt.Fprintf(os.Stderr, "  serve         web dashboard (Bootstrap + WebSocket)\n")