chore: prepare 0.2.1 release

[adamcavendish]

Summary

Prepare OpenAPI Nexus 0.2.1 for release. This promotes the accumulated response-header and uniform Rust API-call error notes, bumps workspace metadata, and points stable installer documentation at the new version.

Behavior

Release surface	Behavior
Workspace crates	Resolve as version 0.2.1 in Cargo metadata and the lockfile
Changelog	Publish the current Unreleased notes under 0.2.1
Stable installers	Download release artifacts from the 0.2.1 GitHub release
Release artifacts	Continue to be planned and built by GitHub Actions

Compatibility

This release includes the documented generated Rust API change where ApiError::headers() returns the backend-native header map and successful response structs expose native headers. Generated clients should be regenerated when adopting 0.2.1. The common Rust ApiCallError remains additive alongside operation-specific typed errors.

Validation

• cargo metadata --locked --format-version=1 --no-deps
• just fmt-check
• cargo check --workspace
• cargo +1.90 check --workspace
• just clippy
• just test-all — 1,302 tests passed
• just golden-check
• just book
• cargo package --allow-dirty

Documentation

• docs/src/response_headers.md: added a Rust call-site example covering declared-header accessors and native-header fallback on successful and error responses.
• Coverage: response headers now have reference, explanation, and how-to coverage; uniform Rust errors remain documented with direct, thiserror, and SNAFU usage.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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