perf: audit EmbeddingGemma 2 and reuse measured query embeddings

apps/desktop/src/search-state.ts

@@ -1,3 +1,14 @@
+/** Let rapid typing settle before starting the model lane. */
+export const SEMANTIC_SETTLE_MS = 120;
+
+/** A rebuild retains the previous committed generation. Backend readiness is
+ * withdrawn only while a replacement query graph is being prepared. */
+export function semanticAvailable(
+  status: { ready: boolean; generation: number | null } | null,
+) {
+  return Boolean(status?.ready && status.generation !== null);
+}
+
 /** One active request per lane; intermediate keystrokes collapse to the latest.
  * Native work already running cannot be cancelled, so revisions guard every
  * result and retry. Busy responses receive at most one delayed retry. */