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/accel_stream_ctl.go

@@ -0,0 +1,40 @@
+package main
+
+import "sync"
+
+// accelStreamCtl tracks which slaves the host wants to poll for accel (mirrors firmware).
+type accelStreamCtl struct {
+	mu      sync.Mutex
+	enabled map[uint32]struct{}
+}
+
+func newAccelStreamCtl() *accelStreamCtl {
+	return &accelStreamCtl{enabled: make(map[uint32]struct{})}
+}
+
+func (c *accelStreamCtl) Set(clientID uint32, on bool) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	if on {
+		c.enabled[clientID] = struct{}{}
+	} else {
+		delete(c.enabled, clientID)
+	}
+}
+
+func (c *accelStreamCtl) Any() bool {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	return len(c.enabled) > 0
+}
+
+func (c *accelStreamCtl) SyncFromClients(clients []ClientView) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	c.enabled = make(map[uint32]struct{})
+	for _, cl := range clients {
+		if cl.AccelStream {
+			c.enabled[cl.ID] = struct{}{}
+		}
+	}
+}