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.

docs/PLAY_RELEASE_CHECKLIST.md

@@ -54,8 +54,10 @@ action except the items marked **[code]**.
 - [x] **Feature graphic** 1024×500 PNG **[code]** — same script.
 - [ ] At least 2 phone screenshots (up to 8). Three are captured (empty state, scanning,
       results), but **re-shoot them after granting photo access on a fresh install**: the
-      existing ones still show the "Photo access needed" banner — see the caveat in
-      STORE_LISTING.md.
+      existing ones still show the "Photo access needed" banner, and the results capture shows
+      the developer's own photo library. Capture the results screen over the generated demo
+      library instead (`python3 tools/make-demo-photos.py`, then push and index it) — see the
+      caveats in STORE_LISTING.md.
       ⚠️ Play rejects a screenshot whose longer side is more than 2× its shorter side, so a
       1080×2400 phone capture has to be cropped to 1080×2160;
       `tools/make-store-screenshots.sh` does exactly that (status and navigation bars only).