feat(stabilization)!: bytesbuf_io 1.0

[martintmk]

Summary

Adds pending stabilization notes for the bytesbuf_io crate based on O365 Core work item 7688115. The notes record the proposed owned-buffer I/O boundary, current and proposed trait requirements, read and write semantics, ecosystem adapter strategy, and unresolved review items. Also adds an empty docs/DESIGN.md placeholder.

Validation

• just package=bytesbuf_io format
• just package=bytesbuf_io readme
• cargo check -p bytesbuf_io --all-features
• cargo build -p bytesbuf_io --all-features
• cargo test -p bytesbuf_io --all-features
• git diff --check
• GitHub required checks passed, including repository spellcheck and CodeQL.
• Codecov reports 100% project and patch coverage.
• Multi-model review completed with no unresolved findings.

Local spellcheck was attempted, but the installed cargo-spellcheck process exits during startup on this machine. The GitHub spell-check job passed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.0%. Comparing base (d799037) to head (5553dbe).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #644   +/-   ##
=======================================
  Coverage   100.0%   100.0%           
=======================================
  Files         473      473           
  Lines       45493    45493           
=======================================
  Hits        45493    45493           

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