Add checks.x86_64-linux covering fmt, clippy, nextest, the four x86_64
release binaries, and all four cross-compiled aarch64 Pi binaries, plus
a .#ci link farm over them so one nix build exercises everything and Nix
schedules the independent branches in parallel.

Move to a nightly toolchain selected with selectLatestNightlyWith, which
skips nightlies missing any requested component. Nightly enables the
parallel rustc frontend (-Z threads) everywhere and the Cranelift codegen
backend for the dev profile, so clippy and tests get faster codegen while
release binaries keep LLVM.

Collapse the four per-binary cross dependency builds into one per target,
scoped to the two Pi crates so host-only crates stay out of the cross
graph, and drop the second toolchain now that one carries both aarch64
targets.

Add a lean devShells.ci that omits wrangler, doctl and the flashing tools.

Fix a never_loop clippy denial in read_wifi_metrics that the new clippy
gate surfaces; the loop only ever ran one iteration.