chore(lint): pin Rust toolchain to 1.95 and adopt clippy 1.95 suggestions
- Add rust-toolchain.toml to align local and CI Rust versions, eliminating
clippy roulette caused by `dtolnay/rust-toolchain@stable` drift.
- Fix 9 clippy 1.95 findings introduced by Hermes Phase 4-8 modules:
* 4x unnecessary_sort_by -> sort_by_key (with Reverse for desc)
* 3x collapsible_match -> match guards
* 1x while_let_loop -> while let
* 1x useless_conversion -> drop redundant .into_iter()
Jason