Add a generated demo photo library for store screenshots

The results screenshot is a screenshot *of a photo library*, so the committed
capture showed the developer's own photos. tools/make-demo-photos.py draws the
library instead: 22 procedural scenes (sunset, mountains, beach, conifers in
haze, dunes, city skyline, night sky, bokeh) with grain and a vignette, so there
are no people, no third-party content and nothing to license.

The duplicate structure is deliberate, because a folder of unrelated pictures
demonstrates nothing:

- headline group: one scene re-saved at falling quality and size until its
  reclaimable size reaches --target-mb (45 MB). Results are ranked by
  reclaimable bytes, so this group has to out-rank whatever is already on the
  device or a real photo is the first thing back in frame.
- exact pair: two byte-identical files, for the SHA-256 path.
- singles: the other scenes, once each, so the library looks lived-in.

Verified with the app's own engine on the desktop before pushing anything:
22 images -> 2 groups (PERCEPTUAL 14 copies, EXACT 2 copies) with the six
singles left alone. Regeneration is byte-identical, the output path resolves
from __file__ so it runs from any cwd, and nothing is written outside the repo.

demo-photos/ is git-ignored, like fixtures/ and store-assets/raw-screenshots/.
STORE_LISTING.md documents the library, why the headline group has a target
size, the MediaProvider scan_file recipe and the cleanup step; ROADMAP, README
and the release checklist point at it.

.gitignore

@@ -25,3 +25,5 @@ keystore.properties
 # Generated data — regenerated on demand, never committed
 fixtures/
 store-assets/raw-screenshots/
+demo-photos/
+store-assets/screenshots/