feat(data-structures): stabilize Deque

[tomas-zijdemans]

I consider Deque to be feature-complete and the external API stable. We could add more functions like some, every or indexOf, but these are all convenience helpers that are addable post-stabilization as non-breaking additions, so not a reason to hold.

Follow-up to #7083, which deferred perf changes out of scope. This PR reapplies only the subset that is a pure internal refactor: toArray, includes, and #copyBuffer linearized into a two-pass walk. The iterator rewrite and Deque.from fast path from the original are intentionally omitted; they can be proposed separately if warranted.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.43%. Comparing base (a3a6ef6) to head (764b9bd).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7092   +/-   ##
=======================================
  Coverage   94.43%   94.43%           
=======================================
  Files         630      630           
  Lines       50566    50587   +21     
  Branches     8969     8974    +5     
=======================================
+ Hits        47750    47770   +20     
- Misses       2247     2248    +1     
  Partials      569      569           

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[bartlomieju]

LGTM