Write the public surface down, so changing it is a deliberate act

[Rafael-SOWNet]

Removing a public member breaks every consumer at compile time; adding one is a promise that has to be kept for the rest of the major version. Neither shows up in a test run — the suite only calls the API it knows about, so a member that vanishes takes its own tests with it and the summary line stays green.

RS0016/RS0017 used to cover this. #835 retired the settings — they were dead, and that was right — but the guarantee went with them, and 2.0 is the release where its absence costs the most.

What this adds

PublicApiSurfaceTest measures the surface by reflection and compares it against Sources/Tests/UnitTests/Common/PublicApi.txt — 2602 members. A difference fails with both lists spelled out, headed by which direction it went.

To accept an intended change:

AM_UPDATE_PUBLIC_API=1 dotnet test Sources/Tests/UnitTests

and commit the file. The diff is then part of review, which is the point — it is not a chore to keep in sync, it is the record of what was promised and when.

Runs in 13 ms.

Choices worth naming

• Protected members count. A consumer can derive, so they are as much a promise as a public one.
• One assembly, one framework. The surface genuinely differs per target — the generic-math members are net7.0+ only — so this pins the framework the tests run on and claims nothing about netstandard2.0.
• By-ref/array/pointer names are normalised. Their Type.FullName embeds the assembly version, so comparing raw would report every such signature as changed on each version bump.

Verified non-vacuous

A guard that cannot fail is worse than none, so I added a public member on purpose:

The public surface no longer matches PublicApi.txt.
ADDED — a promise for the rest of the major version (1):
  AngouriMath.MathS.CanaryDoNotShip(AngouriMath.Entity) : AngouriMath.Entity

What the audit that prompted this found

I diffed 2.0 against the published 1.4.0 package (netstandard2.0, both surfaces dumped by reflection): 2325 → 2579 members, 34 removals.

All 34 are already named in BREAKING-CHANGES.md — no gap. Twelve did not match by name because the document names them collectively, and correctly: "CompilationProtocol's six converter delegates" is exactly six, and the class that owns the other six is listed.

This PR records the surface as it stands; it asserts nothing about whether that surface is right. Two things there are worth a separate decision, not folded in here:

• 2.0 currently adds 288 public members, of which ~86 are the Core.Transformations layer.
• New public types in 2.0: the 8 transformation types, 7 nodes (Ceilf, Floorf, Gcdf, Maxf, Minf, Modf, Roundf), MathS.Quantum, and UnrecognizedFunctionParseException.

Tests: 6050 passing, 0 failed, 14 skipped.