feat: 2.5D stage 6 — public setting, raised tiles, soft sun, theme-free walls (#649)

- settings.volumetric_view (General settings › Display, third switch) replaces
  the alpha entry, the header projection toggle and the phone-menu item; one
  rule for card, sidebar page and kiosk; editors stay Flat; backend accepts
  only a boolean, support package copies it.
- Raised tiles in 2.5D View: no ring, rounded body min(0.275 D, 0.3 h), edge
  0.1 D with the lab filters, ×1.12 size also in layout/collision, lift
  0.075 D, one floor-shadow layer under all markers (theme × floor table),
  frames hugging tile and edge (Alert > Focus > Selected > Hover), forced
  colours without edge and shadow (src/iso-tiles.ts, styles/iso-tiles).
- Soft sun wash instead of projected Flat wedges (src/iso-sun.ts): same gates
  and windows as sun_rays, length from elevation, parallelogram along the sun,
  tone and streaks by floor lightness, sill line.
- Walls take the user's wall colour (top opaque, sides × .77/.68/.60); theme
  rules for walls/openings/labels removed; furniture uses the Flat stroke rule.
- Smokes smoke_iso_tiles/iso_sun/iso_theme_walls/volumetric_setting, 16 new
  mutants, acceptance scenes STAGE6_ACCEPTANCE_SCENARIOS (golden with their
  Linux CI baselines), ACCEPTANCE.md side by side.

Issue: #649
User-Visible: yes

docs/data-hp-contract.json

@@ -78,7 +78,7 @@
       "elements": ["button"],
       "since": "1.78.0-beta.3",
       "audience": ["styling", "test"],
-      "attributes": { "data-id": ["mode-plan", "mode-devices", "mode-decor", "space-settings", "space-add", "settings", "pdf", "support", "summary-settings", "summary-toggle", "projection"] }
+      "attributes": { "data-id": ["mode-plan", "mode-devices", "mode-decor", "space-settings", "space-add", "settings", "pdf", "support", "summary-settings", "summary-toggle"] }
     },
     "header-menu-scrim": {
       "elements": ["div"],
@@ -121,11 +121,6 @@
       "since": "1.73.0-beta.7",
       "audience": ["test"]
     },
-    "projection-toggle": {
-      "elements": ["button"],
-      "since": "1.63.0",
-      "audience": ["styling", "test"]
-    },
     "room": {
       "elements": ["path", "polygon", "rect"],
       "since": "1.59.0-beta.3",