The Area traits are complete and nothing consumes them

`[Trait("Area", …)]` is on **230 of 252** test files, 12 area values, over 3,010 `Fact`/`Theory`. It
was the headline half of #783 and it landed.

**Nothing consumes it.** No workflow passes `--filter`; `CSharpTest` runs the whole suite across an
OS matrix, as it did before the traits existed. And nothing enforces the attribute either, so the 22
files without one will quietly become more.

So the mechanism is in place and unexercised, which is a state worth either finishing or recording:

**Either** shard CI by area — `--filter "Area=Core"` and so on — which is what the attribute was for
and would cut the wall-clock of a matrix leg;

**or** say plainly in `AGENTS.md` that the traits exist for running a subset locally and that CI
deliberately runs everything, so the next person does not read the attribute as load-bearing and
build on it.

Adding a test that fails when a test file has no `Area` trait is cheap and worth doing under either
answer, since the value of the taxonomy is entirely in its completeness.

Filed out of #783, which closed as adopted; this is the rider rather than a reason to reopen it.

Start with the CSharpTest entry point and the CI workflow, then inspect the 230 test files using [Trait("Area", …)] and the 22 without it. Decide whether CI should pass area filters or whether AGENTS.md should document local-only use, and add coverage for missing Area traits; done means the chosen behavior is documented or exercised and taxonomy gaps are prevented.

Written by the indexing model from the issue text.