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.

backend/events/noemit_test.go

@@ -8,9 +8,12 @@ import (
 )
 
 // allowedEmitters are the only files permitted to call the Wails
-// runtime's event emitter directly.
+// runtime's event emitter directly.  There is one call and it lives in
+// runtime_wails.go rather than emit.go because naming the application
+// package is what forces cgo, and cmd/indexbuild builds this package
+// without it.
 var allowedEmitters = map[string]bool{
-	filepath.Join("backend", "events", "emit.go"): true,
+	filepath.Join("backend", "events", "runtime_wails.go"): true,
 }
 
 // TestNoDirectRuntimeEmits fails if anything outside backend/events