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/encoder/mod.rs

@@ -10,7 +10,7 @@ mod raw;
 pub use raw::RawEncoder;
 
 use crate::capture::CapturedFrame;
-use crate::proto::{DirtyRect as ProtoDirtyRect, RawFrame, VideoFrame};
+use crate::proto::VideoFrame;
 use anyhow::Result;
 
 /// Encoded frame ready for transmission
@@ -23,6 +23,8 @@ pub struct EncodedFrame {
     pub size: usize,
 
     /// Whether this is a keyframe (full frame)
+    // Set by encoders; not yet read by the transmit path.
+    #[allow(dead_code)]
     pub is_keyframe: bool,
 }
 
@@ -32,9 +34,11 @@ pub trait Encoder: Send {
     fn encode(&mut self, frame: &CapturedFrame) -> Result<EncodedFrame>;
 
     /// Request a keyframe on next encode
+    #[allow(dead_code)]
     fn request_keyframe(&mut self);
 
     /// Get encoder name/type
+    #[allow(dead_code)]
     fn name(&self) -> &str;
 }
 
@@ -44,9 +48,7 @@ pub fn create_encoder(codec: &str, quality: u32) -> Result<Box<dyn Encoder>> {
         "raw" | "zstd" => Ok(Box::new(RawEncoder::new(quality)?)),
         // "vp9" => Ok(Box::new(Vp9Encoder::new(quality)?)),
         // "h264" => Ok(Box::new(H264Encoder::new(quality)?)),
-        "auto" | _ => {
-            // Default to raw for now (best for LAN)
-            Ok(Box::new(RawEncoder::new(quality)?))
-        }
+        // "auto" and any unknown codec default to raw for now (best for LAN)
+        _ => Ok(Box::new(RawEncoder::new(quality)?)),
     }
 }