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/viewer/input.rs

@@ -12,9 +12,9 @@ use tracing::trace;
 use windows::{
     Win32::Foundation::{LPARAM, LRESULT, WPARAM},
     Win32::UI::WindowsAndMessaging::{
-        CallNextHookEx, DispatchMessageW, GetMessageW, PeekMessageW, SetWindowsHookExW,
-        TranslateMessage, UnhookWindowsHookEx, HHOOK, KBDLLHOOKSTRUCT, MSG, PM_REMOVE,
-        WH_KEYBOARD_LL, WM_KEYDOWN, WM_KEYUP, WM_SYSKEYDOWN, WM_SYSKEYUP,
+        CallNextHookEx, DispatchMessageW, PeekMessageW, SetWindowsHookExW, TranslateMessage,
+        UnhookWindowsHookEx, HHOOK, KBDLLHOOKSTRUCT, MSG, PM_REMOVE, WH_KEYBOARD_LL, WM_KEYDOWN,
+        WM_KEYUP, WM_SYSKEYDOWN, WM_SYSKEYUP,
     },
 };
 
@@ -28,7 +28,9 @@ static INPUT_TX: OnceLock<mpsc::Sender<InputEvent>> = OnceLock::new();
 static mut HOOK_HANDLE: HHOOK = HHOOK(std::ptr::null_mut());
 
 /// Virtual key codes for special keys
+// Several entries are reserved for upcoming special-key fidelity work.
 #[cfg(windows)]
+#[allow(dead_code)]
 mod vk {
     pub const VK_LWIN: u32 = 0x5B;
     pub const VK_RWIN: u32 = 0x5C;