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/UX-MODES.md

@@ -21,7 +21,8 @@ admin — the editor X slot of #647. The card title is hidden. Everything else i
 an item of the gear's menu, in a fixed order: the three editors (current one
 marked), Configure space, Add space (not with a fixed space), General settings,
 Save as PDF, Help and feedback; in View also the summary-panel settings and
-show/hide pair and, with `hp_alpha`, the volumetric/flat toggle. An item does
+show/hide pair (2.5D has no card toggle since #649: it is a General settings
+switch). An item does
 exactly what the button it replaces does and closes the menu; Escape closes it
 and returns focus to the gear; a tap outside only closes it and never reaches
 the plan. A user without write access sees just the summary-panel items. Wider