fix: update Windows pixel loops for Clippy

[bigduu]

Summary

• replace the two Windows BGRA iteration sites flagged by Rust 1.98 Clippy with fixed-size slice chunking
• keep the existing pixel conversion and safety behavior unchanged
• unblock the Windows cross-check without expanding the README documentation PR

Closes #27

Test Plan

• cargo fmt --all -- --check
• cargo clippy --all-targets -- -D warnings
• cargo test --all-targets
• cargo check --target aarch64-pc-windows-msvc --all-targets
• cargo clippy --target aarch64-pc-windows-msvc --all-targets -- -D warnings

[bigduu]

Independent exact-head agent review: APPROVE. Reviewed base e1c57f6 and head 426503e. No P0-P3 findings. Both Windows BGRA pixel loops preserve the old complete-four-byte grouping, remainder behavior, channel order, and WGC row-pitch handling. The diff is exactly +2/-2 in one file, raises no effective MSRV, and adds no compatibility or architectural layer. Rust 1.97 Windows cross-check/strict Clippy and all exact-head GitHub checks passed.