Document generated struct evolution policy

[Yong-yuan-X]

Summary

Refs #202.

• Add a generated struct evolution policy note to owned message, eager view, and lazy view struct docs.
• Document that generated structs may gain fields and that exhaustive struct literals/destructuring are not covered by buffa's semver guarantees.
• Keep this scoped to the Option 2 direction from the issue discussion: documentation only, no #[non_exhaustive] and no new builder API.
• Regenerate checked-in buffa-types and buffa-descriptor outputs so the new rustdoc note appears in committed generated code.

Testing

• cargo fmt --all --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo check --workspace --all-features
• cargo test -p buffa-codegen comments
• cargo test --workspace -- --skip timestamp_ext::tests::systemtime_roundtrip_post_epoch
• Additional no_std / 32-bit / MSRV / miri checks run locally

Notes

• cargo test --workspace has one Windows-only SystemTime precision failure locally; the workspace test pass succeeds when that single test is skipped.
• Docker is not available in this local environment, so conformance was not run locally.

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[Yong-yuan-X]

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[iainmcgin]

[claude code] Thanks for the PR — this is exactly the Option-2 direction from #202 and the guide bullet is spot on.

After reviewing, we decided the policy reads better as a library-level statement than as text repeated on every generated struct, so I've pushed a maintainer commit that reshapes the change:

• Moved the full policy text to a new # Struct evolution policy section on the buffa::Message trait doc, with one-line cross-references from MessageView and LazyMessageView. The trait docs are where buffa already documents library-level contracts about generated structs (serialization model, thread safety, manual-impl discouragement), so this keeps the policy alongside them.
• Reverted the per-struct codegen note. The main reason: for any proto message without a leading comment, the note became the type's rustdoc summary line, so module indexes and search results showed identical boilerplate for every doc-less type.
• Kept your docs/guide.md bullet, tweaked to explicitly bless Foo { x, ..Default::default() } struct-update syntax and to link to the trait section.
• Merged origin/main (the base predated the changie adoption in changelog: adopt changie fragment files to end CHANGELOG merge conflicts #230) and added a .changes/unreleased/ fragment.

Verified locally: task lint, cargo test -p buffa, cargo doc -p buffa with -D rustdoc::broken_intra_doc_links, and the #struct-evolution-policy HTML anchor.

If we later want point-of-use discoverability on individual structs, a one-line per-struct intra-doc link to this trait section is the natural follow-up — but that can wait for actual usage feedback, same as the Option-3 builders.