Add document-format crate

[TrueDoctor]

No description provided.

[gemini-code-assist]

Code Review

This pull request introduces the document-format crate, which provides a typed handle (Gdd) for the .gdd document format, decoupling on-disk layout from the editor's in-memory runtime types. It implements codecs, export options, path layouts, and session state persistence. The review feedback highlights several critical issues: a randomized history writing order in append_history_deltas that can cause document load failures, a race condition on WASM/OPFS in open_from_archive due to non-blocking writes, a potential runtime panic on WASM from using block_on in garbage_collect, and redundant loads in stream_entries due to duplicate hashes in hashes_from_store.

Important

The consumer version of Gemini Code Assist on GitHub is being sunset. Starting June 18, 2026, new organization installations will be blocked, and all code review activity will officially cease on July 17, 2026.
For more details on the timeline and next steps, please review the Help Documentation.