docs(ibs): document inherent byte serialization API and MSRV

[dobby-coder]

Summary

Adds a Public API section to docs/repos/ibs.md covering the new inherent to_bytes / from_bytes methods on PublicKey, SecretKey, UserSecretKey, and Signature, with sizes, encoding layouts, and pinned source snippets. Also clarifies the serde feature row (no longer required for byte serialization), records MSRV 1.65, and notes the new PartialEq / Eq derives.

Source for snippets is pinned at ibs commit c0fd27f2ecacbc81a60104b7e030d39e4780c605 (merge of ibs#9).

Closes #47.

Verification

• npm run docs:build succeeds locally.
• All four pinned source URLs return HTTP 200.
• Banned-words / banned-transitions grep is clean. Em dashes: 0 added (one pre-existing in unrelated section was rewritten). Bold: 0 added.

Reviewer quickstart:

git fetch origin && git checkout docs/ibs-public-api && npm install && npm run docs:dev