Fix CI: modernize workflow, fix clippy/sort violations, repair no_std build

[lthiery]

CI has been red since June 2025. The job died in under a minute — before clippy or tests ever ran — at the `cargo install cargo-sort` step, and the workflow also leaned on two archived actions (`actions-rs/clippy-check@v1`, `styfle/cancel-workflow-action@0.9.1`) running deprecated Node runtimes.

Workflow

• `cargo-sort` installed via `taiki-e/install-action` (prebuilt binary, seconds instead of a from-source build)
• archived actions replaced: plain `cargo clippy` invocation + a `concurrency` group for run cancellation
• `cargo sort --check` now gets `--workspace` — without it, only the virtual root manifest was checked, i.e. nothing
• clippy gets `--all-targets` (lint tests too), tests run with `--all-features` (exercises serde)
• new step: `cargo check -p humanbyte --no-default-features`

Code fixes the tightened CI required

• two `MB as u64` unnecessary casts in bytescale tests (clippy `-Dclippy::all` with `--all-targets`)
• member `Cargo.toml`s sorted
• no_std was broken: `f64::ln()` doesn't exist in `core`, so `humanbyte` without the `std` feature never compiled. The unit exponent is now computed with an exact integer loop (identical display output — all existing display assertions pass unchanged). Also dropped the duplicate `#![no_std]`/`cfg_attr` attribute pair.

All steps verified locally: fmt, sort, clippy, tests (12 pass incl. serde), no_std check.