dist-workspace.toml silently ignores unrecognized keys

cargo-dist does not warn or error on unrecognized keys in `dist-workspace.toml`. This caused a typo to silently disable a security feature for every release of a project.

### Example

```toml
[dist]
github-attestions = true # typo: should be "github-attestations"
```

`dist generate-ci`, `dist plan`, and `dist build` all succeed without any warning. The generated `release.yml` simply omits the attestation step and permissions, as if the key was never set. We shipped multiple releases before discovering the issue by manually diffing against another project's workflow.

### Expected behavior

cargo-dist should warn (or error) when it encounters an unrecognized key in `dist-workspace.toml`. Something like:

```
WARN unknown key `github-attestions` in [dist] — did you mean `github-attestations`?
```

### Context

- cargo-dist 0.30.3
- The typo went undetected across several releases because nothing in the pipeline flagged it
- The `dist plan` CI check (as used by some projects to validate their release config) also does not catch this
- Fixed via: https://github.com/spinel-coop/rv/pull/576

Start by tracing how dist-workspace.toml is parsed and how the dist generate-ci, dist plan, and dist build entry points handle unknown keys. Reproduce the github-attestions example, then verify that an unrecognized key produces a warning or error and that valid github-attestations behavior remains unchanged.

Written by the indexing model from the issue text.