Artwork: reduce false-positive icons and covers with confidence-based fallbacks #18

Observed

During compact-library checks on Android 15 emulator-5554, artwork false positives became conspicuous at large display sizes:

• 2048 (Jan Smucr 1.04): Gallery uses a congratulations text image instead of a title/cover.
• BilliardBerzerk: Grid shows a wood texture as the enlarged icon.
• CavernsOfFire: Grid shows a publisher splash rather than a recognizable game icon.

These are observations in the locally imported demo library, not proof of the exact extraction path/cause. Inspect manifest icons, cached art and candidate scores before changing heuristics.

Scope

Improve the shared artwork selection/fallback pipeline, not a game-name allowlist. Respect explicit valid MIDlet icons; avoid presenting arbitrary backgrounds, text panels, sprite sheets or textures as icons/covers. When confidence is low, use an honest generated fallback. Preserve user-selected artwork and offer a menu-level way to replace/reset automatically chosen art.

Acceptance

• Test a varied local corpus including icon-less JARs and small genuine MIDlet icons.
• Gallery uses a plausible cover or icon-based fallback; List/Grid never magnify an unrelated texture as the game icon.
• Explicit user artwork is not overwritten by automatic rescans.
• Add focused ranking/fallback regression fixtures without commercial game archives.

Separate from #17 (compact layout). No per-game emulator patches are requested.