Repair module aliasing, and stabilize socket on CI

[tonyarnold]

SwiftPM loses a dependent module alias, UDP sends can use invalid source addresses, and reused socket descriptors can lose readiness notifications. Cancellation tests also fail when task startup exceeds their fixed sleeps.

• Preserve both module aliases through the existing conditional CSystemLinux dependency.
• Initialize packet info and read source addresses from concrete address storage.
• Refresh epoll/kQueue registrations after descriptor reuse.
• Synchronize cancellation tests explicitly, preserving cancellation before and after continuation creation.

Includes Luke Howard's fixes from #243 and #244 so that I can test if this is all passing on CI.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.74%. Comparing base (bd32d43) to head (8a7181c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #247      +/-   ##
==========================================
+ Coverage   93.72%   93.74%   +0.01%     
==========================================
  Files          72       72              
  Lines        3777     3787      +10     
==========================================
+ Hits         3540     3550      +10     
  Misses        237      237              

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