Fix README compatibility packages example to use Microsoft.Bcl.Async on net40

[Copilot]

The Microsoft.Bcl.AsyncInterfaces example was misleading: PolyShim doesn't polyfill IAsyncEnumerable<T> when that package is present (the package provides it directly), so no additional PolyShim polyfills were actually being enabled.

Changes

• Replaces the netstandard2.0 + Microsoft.Bcl.AsyncInterfaces / Task.WhenEach example with a net40 + Microsoft.Bcl.Async example
• Microsoft.Bcl.Async sets FEATURE_TASK=true, which unlocks PolyShim's ValueTask (FEATURE_TASK && !FEATURE_VALUETASK) and IAsyncEnumerable<T> (FEATURE_TASK && !FEATURE_ASYNCINTERFACES) polyfills — a genuine case of a compatibility package expanding PolyShim's coverage

// Microsoft.Bcl.Async is referenced, so these polyfills are enabled
static async ValueTask<int> SumAsync(IAsyncEnumerable<int> source)
{
    var sum = 0;
    await foreach (var value in source)
        sum += value;

    return sum;
}

[Copilot]

Pull request overview

Updates the documentation example for compatibility packages to demonstrate a real case where adding a Microsoft compatibility package expands PolyShim’s available polyfills (instead of disabling them due to already-provided APIs).

Changes:

• Replaces the netstandard2.0 + Microsoft.Bcl.AsyncInterfaces example with a net40 + Microsoft.Bcl.Async example.
• Updates the sample code to show ValueTask + IAsyncEnumerable<T> usage enabled via Microsoft.Bcl.Async (i.e., FEATURE_TASK becomes available on net40).

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.75%. Comparing base (b55d189) to head (7d0443c).
⚠️ Report is 2 commits behind head on prime.

Additional details and impacted files

@@            Coverage Diff             @@
##            prime     #116      +/-   ##
==========================================
+ Coverage   90.66%   90.75%   +0.08%     
==========================================
  Files         282      282              
  Lines        4898     4898              
  Branches      401      401              
==========================================
+ Hits         4441     4445       +4     
+ Misses        367      364       -3     
+ Partials       90       89       -1     

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