fix(kble-socket): compile with features = ["stdio"] alone

[sksat (sksat)]

What

Fixes a pre-existing feature-flag bug: kble-socket with features = ["stdio"] (without tungstenite) failed to compile.

The bug

lib.rs re-exported from_stdio whenever stdio was enabled:

#[cfg(feature = "stdio")]
pub use stdio::{from_stdio, Stdio};

But from_stdio is #[cfg(feature = "tungstenite")] (it builds a WebSocketStream). So stdio alone gave error[E0432]: unresolved import stdio::from_stdio.

Fix

Gate the from_stdio re-export on both features; stdio alone still provides the Stdio byte-stream adapter:

#[cfg(feature = "stdio")]
pub use stdio::Stdio;
#[cfg(all(feature = "stdio", feature = "tungstenite"))]
pub use stdio::from_stdio;

Notes

• Pre-existing, unrelated to feat(kble-socket): cancellable pipe stdin; drop per-plug process::exit workarounds #189 (cancellable stdin). Split out per review so feat(kble-socket): cancellable pipe stdin; drop per-plug process::exit workarounds #189 stays focused; touches only lib.rs, so no conflict with feat(kble-socket): cancellable pipe stdin; drop per-plug process::exit workarounds #189.
• The kble plugs all enable stdio + tungstenite, so this changes nothing for them — it just unbreaks the stdio-only combo.
• Verified: cargo build -p kble-socket --no-default-features --features stdio and --features stdio,tungstenite both succeed; full cargo test green.
• Minor: a stdio-only build still emits an unused-import warning for from_stdio's types (in stdio.rs); not hit in CI (which builds stdio+tungstenite), can be tidied alongside the stdio.rs changes.

🤖 Generated with Claude Code

[Copilot]

Copilot encountered an error and was unable to review this pull request. You can try again by re-requesting a review.

[Copilot]

Copilot encountered an error and was unable to review this pull request. You can try again by re-requesting a review.

[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated no new comments.

[sksat (sksat)]

The feature-powerset CI guard added in #191 correctly catches the bug this PR fixes.

Running #191's new cargo hack check --feature-powerset against main (i.e. without this fix), the verify-crate job fails at exactly the stdio-only combination:

running `cargo check --no-default-features --features stdio` on kble-socket (7/15)
error[E0432]: unresolved import `stdio::from_stdio`
   |
13 | pub use stdio::{from_stdio, Stdio};
   |                 ^^^^^^^^^^ no `from_stdio` in `stdio`
error: could not compile `kble-socket` (lib) due to 1 previous error

(run: https://github.com/arkedge/kble/actions/runs/26989996812/job/79647909232)

Every other step (clippy / fmt / install cargo-hack) passes; only the new check feature powerset step is red, on the single combination this PR repairs. Once this PR merges and #191 is rebased onto main, all 15 workspace feature combinations compile and #191 goes green — so #191 both proves and guards against this regression.