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/capture/mod.rs

@@ -9,7 +9,7 @@ mod dxgi;
 #[cfg(windows)]
 mod gdi;
 
-pub use display::{Display, DisplayInfo};
+pub use display::Display;
 
 use anyhow::Result;
 use std::time::Instant;
@@ -33,6 +33,8 @@ pub struct CapturedFrame {
     pub display_id: u32,
 
     /// Regions that changed since last frame (if available)
+    // Populated by capturers; not yet consumed by the encoder pipeline.
+    #[allow(dead_code)]
     pub dirty_rects: Option<Vec<DirtyRect>>,
 }
 
@@ -53,9 +55,11 @@ pub trait Capturer: Send {
     fn capture(&mut self) -> Result<Option<CapturedFrame>>;
 
     /// Get the current display info
+    #[allow(dead_code)]
     fn display(&self) -> &Display;
 
     /// Check if capturer is still valid (display may have changed)
+    #[allow(dead_code)]
     fn is_valid(&self) -> bool;
 }