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/bin/sas_service.rs

@@ -4,7 +4,6 @@
 //! The agent communicates with this service via named pipe IPC.
 
 use std::ffi::OsString;
-use std::io::{Read, Write as IoWrite};
 use std::sync::mpsc;
 use std::time::Duration;
 
@@ -84,6 +83,8 @@ extern "system" {
     ) -> i32;
 }
 
+// Field names mirror the Win32 SECURITY_ATTRIBUTES ABI struct.
+#[allow(non_snake_case)]
 #[repr(C)]
 struct SECURITY_ATTRIBUTES {
     nLength: u32,