style(agent): clear 77 pre-existing clippy -D warnings

CI never ran clippy on the agent crate (the build-server clippy job is
Linux-only and can't compile the Windows agent; build-agent only runs cargo
build), so 77 clippy -D-warnings errors had accumulated. Behavior-preserving
cleanup, code-reviewed APPROVED, locally verified (cargo clippy --workspace
--all-targets --all-features -- -D warnings exits 0; cargo test --workspace =
57 passed).

- let _ = on Win32 resource-teardown BOOL returns (gdi.rs); fallible
  BitBlt/GetDIBits stay error-handled
- removed unused imports/vars; idiom fixes (div_ceil, is_null, transmute
  annotations, match collapsing, useless_conversion)
- #[allow(dead_code)] + comment on genuine Task-6/7 scaffolding (vk consts,
  SpecialKey emission, SAS mgmt API, modifier tracking, GDI frame-diff fields)
- Cargo.lock: cargo pruned ~147 stale transitive entries (no version changes)

Follow-up: add cargo clippy -D warnings to the build-agent CI job so the agent
crate stays clippy-clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

agent/src/transport/websocket.rs

@@ -82,7 +82,7 @@ impl WebSocketTransport {
 
         // Send as binary WebSocket message
         stream
-            .send(WsMessage::Binary(buf.into()))
+            .send(WsMessage::Binary(buf))
             .await
             .context("Failed to send message")?;
 
@@ -132,6 +132,7 @@ impl WebSocketTransport {
     }
 
     /// Receive a message (blocking)
+    #[allow(dead_code)]
     pub async fn recv(&mut self) -> Result<Option<Message>> {
         // Return buffered message if available
         if let Some(msg) = self.incoming.pop_front() {
@@ -164,7 +165,7 @@ impl WebSocketTransport {
                     .context("Failed to decode protobuf message")?;
                 Ok(Some(msg))
             }
-            WsMessage::Ping(data) => {
+            WsMessage::Ping(_data) => {
                 // Pong is sent automatically by tungstenite
                 tracing::trace!("Received ping");
                 Ok(None)
@@ -193,6 +194,7 @@ impl WebSocketTransport {
     }
 
     /// Close the connection
+    #[allow(dead_code)]
     pub async fn close(&mut self) -> Result<()> {
         let mut stream = self.stream.lock().await;
         stream.close(None).await?;