Use FoundationEssentials in CancellableQueue when available

[lhoward]

Motivation

CancellableQueue.swift is the only file in AsyncQueue that does a plain import Foundation. On Linux that single import pulls libFoundation.so → libFoundationInternationalization.so → _FoundationICU into the linked binary via the autolink directive — even though the file only uses UUID and DispatchQueue, and no ICU/internationalization functionality is ever called.

For size-constrained / embedded Linux targets this drags in the entire ICU data blob (tens of MB of flash) and the Foundation internationalization library for no functional benefit.

Change

#if canImport(FoundationEssentials)
import FoundationEssentials   // UUID
import Dispatch               // DispatchQueue
#else
import Foundation
#endif

• UUID is provided by FoundationEssentials.
• DispatchQueue comes from Dispatch (already imported directly in Utilities/Delivery.swift, so this matches existing style in the package).
• Platforms without FoundationEssentials fall back to full Foundation exactly as before — no behavior change.

No public API change; CancellableQueue behaves identically. This just lets it link without full Foundation/ICU where FoundationEssentials exists.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (06b1cfe) to head (84cea56).

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #76   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           13        13           
  Lines         1301      1301           
=========================================
  Hits          1301      1301           

Files with missing lines	Coverage Δ
Sources/AsyncQueue/CancellableQueue.swift	100.00% <100.00%> (ø)

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[dfed]

Thank you for the contribution! One nit before I merge assuming CI passes.