refactor: extract BatchFontStore into new FetchFontsByIds feature

The byId font fetch was a verb-oriented capability with a single
consumer driven by a feature need (materializing comparison picks).
That shape belongs at the feature layer, not on the entity.

Move:
- entities/Font/model/store/batchFontStore -> features/FetchFontsByIds/model/store/fontsByIdsStore
- Class BatchFontStore -> FontsByIdsStore

entities/Font retains the transport primitives (fetchFontsByIds,
seedFontCache) and the keyspace (fontKeys); the feature wraps them in
the reactive store. comparisonStore now imports FontsByIdsStore from
the new feature. The proxy API is imported via direct path so vi.spyOn
on the source module still observes the call.

src/entities/Font/api/index.ts

@@ -9,6 +9,7 @@ export {
     fetchFontsByIds,
     fetchProxyFontById,
     fetchProxyFonts,
+    seedFontCache,
 } from './proxy/proxyFonts';
 export type {
     ProxyFontsParams,