[AZ-659] [AZ-660] [AZ-661] Implement frame publisher + gRPC detection client

AZ-659: FramePublisher with per-consumer drop accounting (Arc<Bytes>
zero-copy fan-out). Adds ConsumerId enum, PublisherStats, FrameReceiver
wrapper, and publisher integration tests (AC-1, AC-2, AC-3).

AZ-660: Bi-directional tonic gRPC stream to ../detections. Reconnect
with bounded exponential backoff (1 s → 30 s cap). Drop-oldest
in-flight budgeting (max_concurrent_in_flight = 2). ai_locked frame
skipping. Integration tests against fixture in-process server
(AC-1: happy path 30 fps/10 s, AC-2: reconnect, AC-3: budget drops,
AC-4: ai_locked skipping).

AZ-661: Schema validation (hard SchemaMismatch error on version
mismatch), model_version latch with ModelVersionChanged events,
sliding-window p99 latency tracker with Tier1Degraded/Tier1Recovered
transitions. Integration tests (AC-1, AC-2, AC-3).

Also: update module-layout.md for frame_ingest and detection_client
to reflect the streaming API shape; code review report batch_18.

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

crates/detection_client/build.rs

@@ -0,0 +1,19 @@
+//! AZ-660 build-time codegen for the `../detections` gRPC contract.
+//!
+//! Mirrors the `telemetry_stream` build script: uses
+//! `protoc-bin-vendored` so the build is self-contained (no system
+//! protoc install required on dev or CI). The PROTOC env var is set
+//! before invoking `tonic-prost-build`.
+
+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/detections.proto"], &["proto"])?;
+
+    println!("cargo:rerun-if-changed=proto/detections.proto");
+    Ok(())
+}