[AZ-675] telemetry_stream Tonic gRPC server + per-client lossy queue

Pins operator-link transport to gRPC server-streaming (closes
architecture Q2 in favour of gRPC). Adds first-time tonic / prost /
tonic-build infrastructure to the workspace; uses
protoc-bin-vendored so neither dev machines nor CI need system
protoc installed.

Design — back-pressure lives in the per-topic tokio::sync::broadcast
ring, drained directly by the tonic-streamed response via
BroadcastStream + StreamMap. No intermediate mpsc buffer that could
absorb back-pressure invisibly. Slow client overrun -> Lagged(n)
event -> per-(client_id, topic) drop counter incremented; healthy
clients on the same topic are unaffected.

Service surface — Subscribe(SubscribeRequest) -> stream
TelemetryMessage; five topics (TelemetrySample, GimbalState,
DetectionEvent, MovementCandidate, MapObjectsBundle); empty topics
list defaults to subscribe-all; empty client_id rejected; stream
drop decrements subscribed_clients via StreamGuard. TelemetrySink
push_detections is now real; push_frame still NotImplemented(AZ-676
video path).

Tests — 6 unit + 5 integration (AC-1..AC-3 via in-process gRPC
client, plus subscribe-all default + empty-client_id rejection).
Clippy on telemetry_stream clean.

Pre-existing mission_executor ac3 test polling race surfaces more
reliably under the new tonic build pressure; documented as
_docs/_process_leftovers/2026-05-20_mission_executor_ac3_flake.md
and unchanged by this batch.

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

crates/telemetry_stream/build.rs

@@ -0,0 +1,19 @@
+//! Build-time codegen for the AZ-675 telemetry gRPC contract.
+//!
+//! We use `protoc-bin-vendored` so the build is self-contained — no
+//! system protoc install is required on dev or CI. The PROTOC env
+//! var is set before invoking `tonic-prost-build`, which is what
+//! tonic uses to locate the compiler.
+
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+    let protoc = protoc_bin_vendored::protoc_bin_path()?;
+    std::env::set_var("PROTOC", protoc);
+
+    tonic_prost_build::configure()
+        .build_client(true)
+        .build_server(true)
+        .compile_protos(&["proto/telemetry.proto"], &["proto"])?;
+
+    println!("cargo:rerun-if-changed=proto/telemetry.proto");
+    Ok(())
+}