perf(open): answer "are there any entities?" with one directory read

The 7.x-to-8.0 layout probe runs on the open path of every store that does not
yet carry its completion marker — a restore, a store built by an older release
— and asked whether the canonical tree holds anything by LISTING it: a
recursive walk of every file in every entity directory, to learn a boolean. It
now asks the one-level door added for generation discovery, falling back to the
listing on an adapter that lacks it.

Also files a defect found while ratifying the operator set: the VFS builds its
path-prefix filter as `$startsWith`, an operator no engine spelling accepts,
so vfs.searchFiles({ path }) throws INVALID_QUERY on every call that passes a
path. Pre-existing, unrelated to the operator work, and left as a filing —
a path-prefix search needs a design answer, not a spelling correction.

src/brainy.ts

@@ -16732,8 +16732,19 @@ export class Brainy<T = any> implements BrainyInterface<T> {
       if (legacyEntityPaths.length === 0) {
         // Already flat (root entities, no head-branch entities) → stamp the marker
         // so future opens short-circuit. A genuinely empty/fresh dir gets no marker.
-        const rootEntities = await probe.listRawObjects('entities')
-        if (rootEntities.length > 0) {
+        // "Are there any entities?" is answered by ONE directory read, not by a
+        // recursive listing of every file in the tree: this runs on the open path
+        // of every store that does not yet carry the marker (a restore, a store
+        // built by an older release), and on a large store that listing walks the
+        // whole canonical tree to learn a boolean.
+        const oneLevel = (
+          probe as unknown as { listRawPrefixes?: (prefix: string) => Promise<string[]> }
+        ).listRawPrefixes
+        const hasRootEntities =
+          typeof oneLevel === 'function'
+            ? (await oneLevel.call(probe, 'entities')).length > 0
+            : (await probe.listRawObjects('entities')).length > 0
+        if (hasRootEntities) {
           await probe.writeRawObject('_system/migration-layout.json', {
             layout: 'flat-v8',
             version: 8,