[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>

Cargo.toml

@@ -62,6 +62,18 @@ reqwest = { version = "0.12", default-features = false, features = ["json", "rus
 jsonschema = { version = "0.18", default-features = false }
 tokio-serial = "5"
 
+# gRPC (operator-link transport — see telemetry_stream / detection_client)
+tonic = "0.14"
+tonic-prost = "0.14"
+prost = "0.14"
+prost-types = "0.14"
+tonic-prost-build = "0.14"
+protoc-bin-vendored = "3"
+tokio-stream = { version = "0.1", features = ["sync", "net"] }
+
+# Lock-free / sync helpers
+parking_lot = "0.12"
+
 # Crypto / hashing
 sha2 = "0.10"