Expose F-Droid artwork and developer metadata for store entries

Resolve localized v1/v2 artwork, retain six ordered screenshots, clean summaries and refresh older source caches. Add offline metadata and cache regression coverage.

Co-Authored-By: GPT-6 Astra <noreply@openai.com>

tests/fixtures/fdroid/README.md

@@ -12,3 +12,10 @@ fingerprint matches the operator's published fingerprint:
 3BF0D6ABFEAE2F401707B6D966BE743BF0EEE49C2561B9BA39073711F628937A.
 It exercises an independent production JAR/CMS encoder without network access.
 The index it references is not needed by this signature-only fixture test.
+
+`artwork-v1.json` and `artwork-v2.json` are unsigned metadata/reducer fixtures
+based on the synthetic indexes above. They exercise en-US preference, per-field
+locale fallback, v1 artwork paths, phone/tablet ordering, the six-image cap,
+author names and HTML/multiline summaries. The signed integrity fixtures remain
+unchanged; artwork tests feed these JSON files directly through the reducer and
+then round-trip the resulting entries through the source cache.