BG: opaque plan paper — the scene background never bleeds through the plan

Owner request 2026-08-03: bg_color (and the daynight sky) used to shine
through the plan itself — a hand-drawn plan's translucent room fills sat
directly on the scene colour, and a transparent backdrop image let it
through too. An opaque rect.hp-paper now sits under everything the plan
draws and hugs the plan's extents (the backdrop image rect, or the drawn
content bounds the opening view fits). Its colour is the pre-bg_color
canvas: white for drawn plans (.stage.noplan), the theme card background
under an image and on the static space-card. The daynight night keeps
dimming the plan via the zoomwrap brightness filter ONLY — the paper's
alpha never changes. The scene colour is visible strictly AROUND the
plan, in view/kiosk/editors and the static card alike.

smoke_bg_color grew the contract (sections 11–13): paper presence,
geometry and opacity in view/editors/night, the white drawn-plan paper,
the static card's paper, plus a pixel proof against an acid #ff00ff
background (screenshot → canvas: no acid admixture inside the plan, acid
right outside it). The suite fails on the previous build.

docs: SUN.md background contract + TESTING.md checklist item.

docs/SUN.md

@@ -61,6 +61,17 @@ only — no entities are created, no services are called.
 - Backend validation: `In(['static', 'daynight'])` at both levels.
 - `'daynight'` follows the general gate: without `north_deg` (or
   without `sun.sun`) it behaves as `'static'`.
+- **The scene background never bleeds through the plan** (owner,
+  2026-08-03). In BOTH modes the background — `bg_color` or the
+  daynight sky — is visible only AROUND the plan: an opaque
+  `rect.hp-paper` sits under everything the plan draws and hugs the
+  plan's extents (the backdrop image rect, or the drawn content bounds
+  the opening view fits). Its colour is the pre-bg_color canvas —
+  white for hand-drawn plans, the theme card background under an
+  image. The night dimming above is the `brightness` filter on the
+  zoomwrap ONLY; the paper's alpha never changes. Applies to
+  view/kiosk/editors and the static space-card alike
+  (smoke_bg_color).
 
 ## Window light wedges — `settings.sun_rays`