ci: check the feature powerset with cargo-hack

[sksat (sksat)]

What

Add a cargo hack check --feature-powerset step to CI so every feature combination of every workspace crate is compiled.

Why

The plugs always pull in kble-socket with both stdio and tungstenite. Thanks to Cargo's feature unification, a normal cargo test over the workspace therefore never builds kble-socket with a single feature on its own — so a crate can advertise a feature that fails to compile by itself and CI stays green.

That is exactly what happened: kble-socket with stdio alone (no tungstenite) has failed to compile since the repo was first opened (7c463ac, 2023-04-12):

error[E0432]: unresolved import `stdio::from_stdio`

from_stdio is #[cfg(feature = "tungstenite")] but was re-exported under #[cfg(feature = "stdio")]. #190 fixes the bug; this PR adds the guard that would have caught it (and will catch the next one).

Demonstrating the catch (RED → GREEN)

cargo hack check --feature-powerset --no-dev-deps -p kble-socket on the current main:

running `cargo check --no-default-features --features stdio` on kble-socket (4/8)
error[E0432]: unresolved import `stdio::from_stdio`
error: could not compile `kble-socket` (lib) due to 1 previous error

With #190's fix applied, all 15 workspace runs (incl. kble-socket's 8 combos) pass.

This PR's own CI is intentionally RED right now — it is running the new check against main, which still has the bug. It goes green once #190 merges and this branch is rebased. That red is the demonstration, not a defect.

Notes

• kble-socket is the only crate with [features], so the powerset is just its 2³ = 8 combinations; other members are checked once (no features). No combinatorial blowup.
• cargo-hack is installed via taiki-e/install-action (a prebuilt binary) rather than cargo install, because the latest cargo-hack's MSRV (1.85) already exceeds this repo's pinned toolchain (1.78). The prebuilt binary is toolchain-independent; the underlying cargo check still runs under the pinned 1.78.

🤖 Generated with Claude Code