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/emit.go

@@ -4,8 +4,6 @@ import (
 	"context"
 	"errors"
 	"log/slog"
-
-	"github.com/wailsapp/wails/v3/pkg/application"
 )
 
 // ErrNoRuntime is returned by Deliver when there is neither a test Sink
@@ -82,16 +80,7 @@ func Deliver(ctx context.Context, name string, data ...any) error {
 		}
 	}
 
-	// application.Get() returns nil when no app is running — under test,
-	// before Run, and after shutdown.  It does not terminate the
-	// process, which is what the v2 probe of the private "events"
-	// context key existed to avoid.
-	app := application.Get()
-	if app == nil {
-		return ErrNoRuntime
-	}
-
-	app.Event.Emit(name, data...)
-
-	return nil
+	// emitRuntime is the only place the Wails application is touched,
+	// and it is behind a build tag: see runtime_wails.go.
+	return emitRuntime(name, data...)
 }

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

backend/events/runtime_indexbuild.go

@@ -0,0 +1,18 @@
+//go:build indexbuild
+
+package events
+
+// emitRuntime is the index tools' half of the split described in
+// runtime_wails.go: under the indexbuild tag there is no Wails
+// application to emit through, and importing the one that would be
+// there costs cgo and a GTK/WebKit toolchain the index build container
+// deliberately does not have.
+//
+// Returning ErrNoRuntime is the same answer the app gives before Run
+// and after shutdown, so Emit's callers need no second code path: an
+// event emitted by cmd/indexbuild is logged and dropped.  A test that
+// wants to observe one installs a Sink with WithSink, which Deliver
+// consults first and which works under either tag.
+func emitRuntime(_ string, _ ...any) error {
+	return ErrNoRuntime
+}

backend/events/runtime_wails.go

@@ -0,0 +1,32 @@
+//go:build !indexbuild
+
+package events
+
+import (
+	"github.com/wailsapp/wails/v3/pkg/application"
+)
+
+// emitRuntime pushes an event through the running Wails application.
+//
+// It is the app's half of a two-file split, and the split exists for a
+// build constraint rather than a design one: v3's application package
+// is cgo on Linux, so anything importing it needs GTK and WebKit
+// headers to compile.  cmd/indexbuild and cmd/indexexport reach this
+// package through backend/explore and are built in a plain golang
+// container with CGO_ENABLED=0 (.gitea/workflows/index-artifact.yml),
+// where that is not available and not wanted.  See
+// runtime_indexbuild.go for the other half.
+func emitRuntime(name string, data ...any) error {
+	// application.Get() returns nil when no app is running — under
+	// test, before Run, and after shutdown.  It does not terminate the
+	// process, which is what the v2 probe of the private "events"
+	// context key existed to avoid.
+	app := application.Get()
+	if app == nil {
+		return ErrNoRuntime
+	}
+
+	app.Event.Emit(name, data...)
+
+	return nil
+}