no-std Step 2: gate readers/ and dataset I/O behind #[cfg(feature = "std")] #415

Sub-task of #413.

What

Isolate all std-only I/O surfaces so they compile out when feature = "std" is not set:

• src/readers/csv.rs: uses std::io::Read — wrap the entire module or the Read-dependent API in #[cfg(feature = "std")].
• src/readers/error.rs: uses std::io error types — same gate.
• src/dataset/mod.rs::serialize_data (lines 57–87): uses std::fs::File — already gated #[cfg(not(target_arch = "wasm32"))]; change to #[cfg(feature = "std")].
• src/dataset/mod.rs::deserialize_data (lines 103–210): returns io::Error — replace with Failed or a custom no-std error enum so the deserialization algorithm remains available on embedded.

Notes

• readers/ is already behind #[cfg(feature = "serde")]; the new std gate stacks on top.
• postcard (the serde backend) supports no_std; verify no std::io types leak through its API at the use sites.
• This is the highest-risk step as it touches public API. Add or update compile-tests to confirm both std and no-std paths build.

Acceptance

• cargo build --no-default-features compiles without pulling in std::io
• cargo build --features "std,serde" still exposes readers/ and dataset I/O as before
• cargo test --all-features still passes

Estimated effort: ~2 hrs