[cberner]

Step 3 of 3: removes redb::io, following #1636 and #1637 (both merged). Rebased onto master; the tree is unchanged.

• Io variants are std-only. The Io variants of BackendError, StorageError and Error, and their From<std::io::Error> / From<BackendError> for std::io::Error impls, now exist only with std (cfg(not(redb_no_std))). They hold std::io::Error directly. All three enums are #[non_exhaustive], so a variant that appears when another crate turns on std can't break a downstream match. Without experimental-api-5, std is always on, so 4.x users see no change.
• src/io.rs is deleted, along with the stand-in io::Error and its tests. Without std, a backend reports failures with BackendError::Other (from Add BackendError::Other for backend failures that are not I/O errors #1637).
• The invalid_input/invalid_data helpers are now crate-private constructors on BackendError. With std they build the same io::Error kinds as before; without std the kind is dropped, as it was before.
• The no_std regression crate now also runs a backend whose sync_data returns BackendError::Other, and checks that create_with_backend reports it as StorageError::Backend.
• README and CHANGELOG no_std notes are updated.

After this, the remaining flag-removal work for this area is mechanical: replace the BackendIoError alias from #1636 with BackendError and drop its useless_conversion allows.

Checked:

• cargo fmt.
• clippy --all --all-targets, with and without --all-features, on Linux, Windows and macOS.
• clippy for thumbv7em (no_std), with and without logging.
• cargo check for wasm32-unknown-unknown (3 feature sets) and wasm32-wasip1.
• cargo doc.
• cargo test --all-features, and cargo test with default features.
• The no_std regression crate.

🤖 Generated with Claude Code

https://claude.ai/code/session_01ALhxet6NP69dCfAUv8TAhR

[chatgpt-codex-connector]

Codex Review Summary: ✅ Completed

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-11-08T21:27:34.965710Z	b0c1fbc	New commits

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review" or "@codex security review".

Codex reacts with 👀 while any review is running, comments if it has suggestions, and reacts with 👍 once all reviews finish with no findings.