build: Add uniflight (name pending) crate for duplicate request coalescing

[schgoo]

Adds uniflight, a crate for coalescing duplicate async tasks into a single execution. When multiple tasks request the same work (identified by a key), only the leader performs the work while followers wait and receive a clone of the result.

• Handles cancellation and panic gracefully (followers become the new leader)
• Includes comprehensive module documentation
• Includes example demonstrating cache thundering herd prevention

[schgoo]

@microsoft-github-policy-service agree company="Microsoft"

[codecov]

Codecov Report

❌ Patch coverage is 98.02956% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 99.9%. Comparing base (32e413b) to head (eac8971).
⚠️ Report is 47 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/uniflight/src/lib.rs	98.0%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #118     +/-   ##
========================================
- Coverage   100.0%   99.9%   -0.1%     
========================================
  Files          33      68     +35     
  Lines        3291    7116   +3825     
========================================
+ Hits         3291    7112   +3821     
- Misses          0       4      +4     

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[geeknoid]

I like the name :-)