fix(build): keep the index tools free of the Wails application

The v3 migration put application.Get() in backend/events and a
ServiceStartup hook in backend/explore, both of which cmd/indexbuild
reaches. v3's application package is GTK/WebKit bindings on Linux, so
the index-artifact job — a plain golang container with CGO_ENABLED=0,
on the stated grounds that neither command imports the app — stopped
compiling with "undefined: pointer". That job owns the ~205 GB dump
checkpoint, so it is the worst place to learn this.

Both are behind the indexbuild tag now: the one app.Event.Emit lives in
runtime_wails.go, runtime_indexbuild.go answers ErrNoRuntime (what the
app itself returns before Run, so Deliver's callers need no second
path), and explore's ServiceStartup moves to its own tagged file.

TestIndexToolsDoNotImportWails walks `go list -deps -tags indexbuild`
so the claim the workflow makes is checked rather than assumed.

cmd/indexbuild/deps_test.go

@@ -0,0 +1,63 @@
+//go:build indexbuild
+
+package main_test
+
+import (
+	"os/exec"
+	"strings"
+	"testing"
+)
+
+// wailsApp is the package that costs cgo: on Linux it is GTK and
+// WebKit bindings, which the index build container does not have.
+const wailsApp = "github.com/wailsapp/wails/v3/pkg/application"
+
+// TestIndexToolsDoNotImportWails fails if cmd/indexbuild or
+// cmd/indexexport reach the Wails application package.
+//
+// .gitea/workflows/index-artifact.yml builds both in a plain golang
+// image with CGO_ENABLED=0, on the stated grounds that neither imports
+// the app.  That stopped being true when the v3 migration put
+// application.Get() in backend/events and a ServiceStartup hook in
+// backend/explore, and the workflow only found out at the point where
+// it is most expensive to find out — the job that owns the 205 GB
+// checkpoint.  Both are behind the indexbuild tag now
+// (backend/events/runtime_wails.go), and this is what keeps them there.
+//
+// It shells out to `go list` because the constraint is about the
+// *linked* dependency graph under a particular tag, which no import of
+// this package can observe.
+func TestIndexToolsDoNotImportWails(t *testing.T) {
+	t.Parallel()
+
+	if _, err := exec.LookPath("go"); err != nil {
+		t.Skip("no go toolchain on PATH")
+	}
+
+	for _, pkg := range []string{
+		"yellowjacket/cmd/indexbuild",
+		"yellowjacket/cmd/indexexport",
+	} {
+		t.Run(pkg, func(t *testing.T) {
+			t.Parallel()
+
+			out, err := exec.Command(
+				"go", "list", "-deps", "-tags", "indexbuild", pkg,
+			).Output()
+			if err != nil {
+				t.Fatalf("go list %s: %v", pkg, err)
+			}
+
+			for _, dep := range strings.Split(string(out), "\n") {
+				if strings.TrimSpace(dep) == wailsApp {
+					t.Errorf(
+						"%s imports %s, so it needs cgo and a GTK/WebKit "+
+							"toolchain; put the dependency behind the "+
+							"indexbuild tag",
+						pkg, wailsApp,
+					)
+				}
+			}
+		})
+	}
+}