Add polyfills for IProgress<T>, Progress<T>, and EventHandler<T>

[Copilot]

Adds IProgress<T>, Progress<T>, and EventHandler<T> polyfills for platforms that lack native support (.NET Framework < 4.5, .NET Standard < 1.1/1.3).

New files

• NetCore10/IProgress.cs — System.IProgress<T> interface polyfill, active on NETFRAMEWORK && !NET45_OR_GREATER or NETSTANDARD && !NETSTANDARD1_1_OR_GREATER
• NetCore10/Progress.cs — System.Progress<T> implementation, active on NETFRAMEWORK && !NET45_OR_GREATER or NETSTANDARD && !NETSTANDARD1_3_OR_GREATER
• NetCore10/EventHandler.cs — System.EventHandler<T> delegate polyfill without the where TEventArgs : EventArgs constraint, active on NETFRAMEWORK && !NET45_OR_GREATER

Notable details

EventHandler<TEventArgs> on net35/net40 enforces TEventArgs : EventArgs. The EventHandler<T> polyfill shadows the constrained BCL delegate for those TFMs, allowing Progress<T>.ProgressChanged to be declared and used with any unconstrained T on all targets.

// Works everywhere, including net35/net40
IProgress<int> progress = new Progress<int>(v => Console.WriteLine(v));
progress.Report(42);

// ProgressChanged is also available on all polyfilled targets
var p = new Progress<int>();
p.ProgressChanged += (_, v) => Console.WriteLine(v);

[Copilot]

Pull request overview

Adds PolyShim type polyfills for progress reporting (System.IProgress<T> and System.Progress<T>) on older target frameworks where these APIs are missing, along with signature list updates and basic behavioral tests.

Changes:

• Add System.IProgress<T> polyfill for .NET Framework < 4.5 and .NET Standard < 1.1.
• Add System.Progress<T> polyfill for .NET Framework < 4.5 and .NET Standard < 1.3 (with ProgressChanged excluded on older .NET Framework where EventHandler<T> is constrained).
• Update Signatures.md and add Progress<T> tests.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
Signatures.md	Updates total/type counts and lists the newly added IProgress<T> and Progress<T> entries.
PolyShim/NetCore10/IProgress.cs	Introduces the System.IProgress<T> interface polyfill under older TFM guards.
PolyShim/NetCore10/Progress.cs	Introduces the System.Progress<T> implementation polyfill using SynchronizationContext.
PolyShim.Tests/NetCore10/ProgressTests.cs	Adds tests covering Report behavior for handler and event scenarios.

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[Tyrrrz]

Can we polyfill that too? @copilot

[Copilot]

Yes — added EventHandler<T> polyfill in 8fb6a2e (PolyShim/NetCore10/EventHandler.cs, guarded to NETFRAMEWORK && !NET45_OR_GREATER) that declares the delegate without the where TEventArgs : EventArgs constraint. With that in place the #if guard is gone from both Progress.cs and the test, and ProgressChanged is now available on all polyfilled TFMs.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.83%. Comparing base (3ccd557) to head (a9f6af6).
⚠️ Report is 1 commits behind head on prime.

Additional details and impacted files

@@            Coverage Diff             @@
##            prime     #120      +/-   ##
==========================================
+ Coverage   90.78%   90.83%   +0.05%     
==========================================
  Files         284      285       +1     
  Lines        4935     4962      +27     
  Branches      404      404              
==========================================
+ Hits         4480     4507      +27     
  Misses        365      365              
  Partials       90       90              

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