Add CACHE_STATUS UART poll and dashboard live stream.

Combine cached accel and tap in one low-overhead master command for ~16 ms
host polling. The dashboard uses a single live-stream toggle plus per-slave
accel-stream controls; fix live_stream state so polling is not cleared every
slow client refresh.

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

main/proto/uart_messages.proto

@@ -27,6 +27,8 @@ enum MessageType {
   BATTERY_STATUS = 26;
   TAP_NOTIFY = 27;
   TAP_SNAPSHOT = 28;
+  /** Combined cached accel + tap poll (one UART round-trip, ~16 ms cadence). */
+  CACHE_STATUS = 29;
 }
 
 message UartMessage {
@@ -63,6 +65,8 @@ message UartMessage {
     TapNotifyResponse tap_notify_response = 30;
     TapSnapshotRequest tap_snapshot_request = 31;
     TapSnapshotResponse tap_snapshot_response = 32;
+    CacheStatusRequest cache_status_request = 33;
+    CacheStatusResponse cache_status_response = 34;
   }
 }
 
@@ -232,6 +236,16 @@ message TapSnapshotResponse {
   repeated TapEvent events = 1 [(nanopb).max_count = 16];
 }
 
+/** Host → master: one-shot read of subscribed cached slave data (no request body). */
+message CacheStatusRequest {}
+
+message CacheStatusResponse {
+  /** Slaves with accel_stream_enabled. */
+  repeated AccelSample accel = 1 [(nanopb).max_count = 16];
+  /** Slaves with any tap notify flag; pending taps are consumed (like TAP_SNAPSHOT). */
+  repeated TapEvent taps = 2 [(nanopb).max_count = 16];
+}
+
 message EspNowUnicastTestRequest {
   uint32 client_id = 1;
   uint32 seq = 2;