feat(valkey_sink): ValkeyEventSink -- first Valkey consumer (deploy-C §5.6)

Implement a bounded-mpsc + background-task Valkey event sink:
- XADD lifecycle events to a capped stream (MAXLEN ~ 10000).
- Fire-and-forget emit(): one mpsc::try_send, count-and-drop on full.
- Arc<AtomicU64> drop counter surfaced in /metrics.
- main.rs selects ValkeyEventSink when RUTSTER_VALKEY_URL is set and
  falls back to TracingEventSink otherwise.
- config::valkey_url validates redis:// / rediss://.
- CI services:valkey: block on test and clippy jobs.

Signed-off-by: Aaron D. Lee <himself@adlee.work>

.github/workflows/ci.yml

@@ -29,6 +29,18 @@ jobs:
 
   clippy:
     runs-on: ubuntu-latest
+    services:
+      valkey:
+        image: valkey/valkey:latest
+        ports:
+          - 6379:6379
+        options: >-
+          --health-cmd "valkey-cli ping"
+          --health-interval 10s
+          --health-timeout 5s
+          --health-retries 3
+    env:
+      VALKEY_URL: redis://127.0.0.1:6379/
     steps:
       - uses: actions/checkout@v4
       - uses: dtolnay/rust-toolchain@stable
@@ -41,9 +53,21 @@ jobs:
 
   test:
     runs-on: ubuntu-latest
+    services:
+      valkey:
+        image: valkey/valkey:latest
+        ports:
+          - 6379:6379
+        options: >-
+          --health-cmd "valkey-cli ping"
+          --health-interval 10s
+          --health-timeout 5s
+          --health-retries 3
     strategy:
       matrix:
         toolchain: [stable, "1.85"]
+    env:
+      VALKEY_URL: redis://127.0.0.1:6379/
     steps:
       - uses: actions/checkout@v4