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/tray/mod.rs

@@ -6,10 +6,10 @@
 //! - End session
 
 use anyhow::Result;
-use muda::{Menu, MenuEvent, MenuItem, PredefinedMenuItem, Submenu};
+use muda::{Menu, MenuEvent, MenuItem, PredefinedMenuItem};
 use std::sync::atomic::{AtomicBool, Ordering};
 use std::sync::Arc;
-use tracing::{info, warn};
+use tracing::info;
 use tray_icon::{Icon, TrayIcon, TrayIconBuilder, TrayIconEvent};
 
 #[cfg(windows)]
@@ -28,7 +28,8 @@ pub enum TrayAction {
 /// Tray icon controller
 pub struct TrayController {
     _tray_icon: TrayIcon,
-    menu: Menu,
+    // Kept alive for the lifetime of the tray icon; not read directly.
+    _menu: Menu,
     end_session_item: MenuItem,
     debug_item: MenuItem,
     status_item: MenuItem,
@@ -86,7 +87,7 @@ impl TrayController {
 
         Ok(Self {
             _tray_icon: tray_icon,
-            menu,
+            _menu: menu,
             end_session_item,
             debug_item,
             status_item,
@@ -124,14 +125,9 @@ impl TrayController {
         }
 
         // Check for tray icon events (like double-click)
-        if let Ok(event) = TrayIconEvent::receiver().try_recv() {
-            match event {
-                TrayIconEvent::DoubleClick { .. } => {
-                    info!("Tray icon double-clicked");
-                    return Some(TrayAction::ShowDetails);
-                }
-                _ => {}
-            }
+        if let Ok(TrayIconEvent::DoubleClick { .. }) = TrayIconEvent::receiver().try_recv() {
+            info!("Tray icon double-clicked");
+            return Some(TrayAction::ShowDetails);
         }
 
         None