[feat](trx-frontend-http): /meter SSE endpoint for instant signal metering

Adds a dedicated /meter SSE stream that wraps the per-rig meter watch
and emits one compact JSON frame per update with no equality gating, so
30 Hz samples reach the browser unthrottled. Registered as a Read-access
route. app.js opens a dedicated EventSource on /meter alongside /events,
writing directly to the signal bar and value on each frame with no
requestAnimationFrame coalescing, starts/stops with connect/disconnect,
and reconnects on rig switch.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
Signed-off-by: Stan Grams <sjg@haxx.space>

Cargo.lock

@@ -3168,6 +3168,7 @@ dependencies = [
  "serde_json",
  "tokio",
  "trx-core",
+ "trx-protocol",
  "uuid",
 ]