The netstandard2.0 public surface and dependencies are checked by nothing

`UnitTests` targets `net10.0` only, so everything asserted about the public surface and about the
kernel's dependencies is asserted about **one** of the three target frameworks.

`Sources/AngouriMath/AngouriMath.csproj` builds `netstandard2.0;net8.0;net10.0`, and the legs differ
by construction:

- `Core/Serialization` is `#if NET8_0_OR_GREATER`
- `GenericMath` is `Compile Remove`d below net7
- `netstandard2.0` carries `System.Memory` as a package rather than a framework reference

So `PublicApi.txt` records the net10.0 surface, and **the netstandard2.0 public surface is checked by
nothing.** A member could be added to, or removed from, that leg without any test noticing.

This is #783's own objection — *"unbuilt configurations rot silently"* — arriving through the one
construct that issue permits. It closed as adopted; this is the piece it did not cover.

`KernelDependenciesTest` (#1181) has the same bound and says so in its own remarks: it asserts the
referenced assemblies of whichever framework `UnitTests` builds, which is why `System.Memory` appears
there as a framework reference and the netstandard2.0 package reference is invisible to it.

## What would close this

A per-TFM check, and the cheap version is probably enough: multi-target `UnitTests` (or a small
second test project) at least across `netstandard2.0` and the current `net`, and record one
`PublicApi.txt` per leg. That also gives `KernelDependenciesTest` its second column for free.

## What is not being claimed

No defect has been observed on the netstandard2.0 leg. The point is that one could not be observed —
`EverythingBuild.yml` compiles it, which catches a *build* break, and nothing checks its surface or
its dependencies. Filed because two separate checks have now hit the same bound and both had to
record it as a limitation rather than test it.

Start with Sources/AngouriMath/AngouriMath.csproj and the UnitTests target framework configuration, then read PublicApi.txt and KernelDependenciesTest to understand the current single-TFM checks. Compare the netstandard2.0 and current net legs, using EverythingBuild.yml as context. Done means each selected TFM has its public API and kernel dependencies checked, with separate PublicApi.txt coverage.

Written by the indexing model from the issue text.