build: fix the red net48 leg with Polyfill instead of a hand-rolled shim

[dborgards]

main is red. windows-latest has failed since #83 merged:

DeferredEchoQueue.cs(46,43): error CS0305: Using the generic type
'TaskCompletionSource<TResult>' requires 1 type arguments

How it got there

#81 and #83 were both green and merged four minutes apart. #81 added DeferredEchoQueue, which uses the non-generic TaskCompletionSource. #83 added the net48 leg that now has to compile it. Neither was ever built against the other: strict_required_status_checks_policy is false on the main ruleset, so GitHub does not re-run a pull request's checks when main moves underneath it. Both green checkmarks were honest about a tree that no longer existed.

The non-generic TaskCompletionSource is .NET 5+. netstandard2.0 and net48 have only TaskCompletionSource<TResult>.

The fix

Rather than adding a second shim to the hand-rolled TaskWaitAsyncPolyfill, this switches to Polyfill 11.3.0, which supplies both the non-generic TaskCompletionSource and Task.WaitAsync. TaskWaitAsyncPolyfill.cs (99 lines) is deleted.

Why the package is safe for a library repository:

• Source only — <developmentDependency>true</developmentDependency>, no lib/ folder. The code compiles in; the package never appears as a dependency of anything shipped. The reference is scoped to the test project with PrivateAssets=all regardless.
• No using directives needed — the package ships global using global::Polyfills; in its own Polyfill.cs, so the extension-method polyfills resolve at every call site.
• LangVersion 12 → 14, because Polyfill's sources use C# 14 extension members (extension(Byte) { … }). The language version is independent of the target frameworks, so the netstandard2.0 assets are unaffected.

Verified locally

check	result
dotnet build -f net48	succeeded, 0 warnings, 0 errors
dotnet build CanKit.Pro.sln -c Release	succeeded, 0 warnings
dotnet test -c Release	409 passed, 0 failed
*.approved.txt diff	empty — the public API surface does not move
dotnet pack	no PolyfillTargetsForNuget warning

Follow-ups, deliberately not here

1. src/ still carries six byte-identical copies of IsExternalInit.cs, one per package, which Polyfill would also supply. That means a PackageReference in the shipping projects, so it belongs in its own pull request rather than in a fix for a red main.
2. This incident is an argument for flipping strict_required_status_checks_policy to true on the main ruleset. It costs a branch update before merging; it would have caught this automatically.

🤖 Generated with Claude Code

[cursor]

PR Summary

Low Risk
Test-only build wiring and a deleted local shim; shipped packages are unchanged aside from a higher C# language ceiling that does not alter emitted library APIs.

Overview
Restores the net48 test leg after it started failing on non-generic TaskCompletionSource (used by DeferredEchoQueue) and on widespread Task.WaitAsync calls that .NET Framework lacks.

The hand-rolled TaskWaitAsyncPolyfill.cs shim is removed. Tests instead reference the source-only Polyfill package (11.3.0) via central versioning and tests/Directory.Build.props, scoped with PrivateAssets=all so nothing shipped picks it up.

Repo-wide LangVersion is raised 12 → 14 so Polyfill’s sources parse (C# 14 extension members); comments note this does not change how existing library code compiles on netstandard2.0.

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