feat(stabilization)!: bytesbuf 1.0

[martintmk]

Summary

Adds pending stabilization notes for the bytesbuf crate based on O365 Core work item 7688114. The notes record the proposed BytesBuf, BytesView, owned-buffer I/O, and memory-provider stable boundary, along with deferred low-level APIs and unresolved review items. Also adds an empty docs/DESIGN.md placeholder.

Validation

• just package=bytesbuf format
• just package=bytesbuf readme
• cargo build -p bytesbuf
• cargo test -p bytesbuf
• git diff --check
• GitHub required checks and Codecov passed
• Spellcheck attempted locally, but the installed cargo-spellcheck process exits during startup on this machine; the GitHub spell-check job passed.

Review

• Multi-model review completed with Opus, Sonnet, and GPT reviewers.
• Review findings about feature gates, memory-provider implementation dependencies, and BlockMeta exposure points were addressed.
• Final review found no remaining substantive issues.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.0%. Comparing base (d799037) to head (cf75956).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #645   +/-   ##
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  Coverage   100.0%   100.0%           
=======================================
  Files         473      473           
  Lines       45493    45493           
=======================================
  Hits        45493    45493           

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