release: prepare 2.1.1

[leehack]

Summary

• Bump mcp_dart to 2.1.1 and promote unreleased notes into the release changelog.
• Update public docs and llms.txt install/release guidance for 2.1.1.
• Document standard JSON Schema enum output and serialized concurrent stdio writes.

Verification

• dart format .
• dart analyze
• dart test
• dart pub publish --dry-run

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.64%. Comparing base (3c4aa6d) to head (8166fb8).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #91   +/-   ##
=======================================
  Coverage   80.64%   80.64%           
=======================================
  Files          43       43           
  Lines        5801     5801           
=======================================
  Hits         4678     4678           
  Misses       1123     1123           

Flag	Coverage Δ
unittests	80.64% <ø> (ø)

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