Four changes, each removing or clarifying surface that was only there for
internal wiring or was speculative:

- Export Upcaster: RegisterEvent's param was an unexported alias (upcastFn),
  so godoc rendered an un-nameable type. Now `type Upcaster = func(...)`.

- Delete StateAppCounter: documented as a shipped specialization but had zero
  consumers (only its own unit test). Speculative surface; dropped until a
  real need appears. docs/distributed-state.md now shows the inline
  StateAppEvents[tick,int64] pattern instead.

- Move WithTestServer out of production into vt.Serve(t, app): the option
  pulled net/http/httptest into production config.go/app.go to serve a
  test-only need. App is its own http.Handler and ServeHTTP dispatches
  dynamically, so httptest.NewServer(app) is equivalent and order-independent.
  Added vt.Serve (auto-closes via t.Cleanup) and migrated all call sites;
  internal `package via` tests use httptest.NewServer(app) directly (vt->via
  import cycle bars vt there).

- Document the sealed readCtx param: StateTab.Text and StateAppEvents.Text now
  state they accept *Ctx or *CtxR, matching the other Read/Text methods, so the
  opaque (intentionally unexported) readCtx in the signature is explained.

Build, vet, gofmt, and `go test -race ./...` all green.