feat(qa): reference-vs-render iteration loop; screenshots are demo-only

Tools/visual_loop.py:
- --verify gates every capture against its expectation in Tools/visual_targets.json
  (missing, blank, tone-less or wrong-size shots fail), so 'each screenshot shows
  what was expected' is checked rather than assumed.
- --compare measures luminance, RMS contrast, edge density, palette distance and a
  3x3 luma grid against the reference photograph, with a 0-1 similarity score.
- --iteration N writes Art/reference/iterations/N/{metrics.json,feedback.md}; the
  feedback names the skill to consult (blender-modeler, archviz, materials, lookdev,
  camera-cinematography), so the next modelling pass starts from a measurement.
- Iterations are gitignored until the owner confirms a shot; only confirmed shots
  are promoted into Screenshots/.

Screenshots/ holds demo-map captures only now: render_screenshots.sh renders just
DemoMap, collects the shots and runs the verification gate (--verify); the review
staging map is no longer rendered for review and its PNGs are removed. The README
is rewritten around the four demo shots with per-shot expected content and a
verified column, and no longer claims the review staging set.

Tests: 67 pass (13 new) covering metrics on synthetic images, comparison scoring,
the feedback rules and the target spec (including that every target is a demo shot
and that iterations are gitignored).

.gitignore

@@ -47,6 +47,10 @@ Thumbs.db
 /Geo/raw/mapbox-cache/
 /Geo/verification/*mapbox*
 
+# Visual-loop iterations: generated comparisons stay uncommitted until the owner
+# confirms a shot. Only Screenshots/ is committed.
+/Art/reference/iterations/
+
 # Local, private reference material — never committed (see Docs/reference-notes-hollywood.md)
 /Art/reference/
 /Docs/data-inventory.md