Панорама двигает сцену трансформом, viewBox пишется по бюджету

Перезапись `viewBox` — это не сдвиг, а инвалидация растеризации всей сцены.
Кадр жеста делал её каждый раз: в профиле владельца (Firefox 155, 144 Гц) кадр
доезжал до экрана 200 мс, а драйвер пропускал 124–144 тика в секунду с пометкой
«ждём краску».

Теперь `paintLiveViewport` держит якорь — кадр, чей `viewBox` записан в DOM, и
момент записи. Кадр жеста двигает узлы сцены тем же проективным преобразованием,
которым уже двигались HTML-слои, а `viewBox` переписывается по бюджету: 100 мс
либо 15 % сдвига/масштаба. Ни атрибут, ни стиль не пишутся, если строка не
изменилась.

Issue: #531
User-Visible: yes

docs/ARCHITECTURE.md

@@ -1914,6 +1914,36 @@ core; known-LQI and solid parent routes remain single strokes. Cache data, IEEE
 addresses and raw payloads are never persisted, logged, exported or admitted
 to support diagnostics.
 
+## Live viewport: a transform per frame, a `viewBox` on a budget (#531, 2026-09-11)
+
+Rewriting the `viewBox` attribute is not a move, it is a repaint: the whole SVG
+scene is re-rasterized. Doing it once per gesture frame is what made panning
+crawl on the owner's machine — the frame reached the screen in 200 ms and the
+refresh driver skipped 124–144 ticks per second marked "waiting for paint".
+
+So `paintLiveViewport` keeps an anchor: the frame whose `viewBox` is currently
+written into the DOM, and when it was written. Every gesture frame moves the
+scene nodes by the same projective transform (`liveLayerProjection`,
+`transform-origin: 0 0`) that already moved the HTML layers — a composited move,
+no repaint. The `viewBox` is rewritten only when `needsViewBoxRefresh` says so:
+`LIVE_VIEWBOX_REFRESH_MS` (100 ms) has passed, or the view shifted by
+`LIVE_VIEWBOX_REFRESH_SHIFT` (15 %) of its own size on either axis, or the scale
+changed by as much. The time budget covers ordinary dragging; the shift budget
+covers a flick, where the plan can travel half a screen before 100 ms is up and
+an empty band on the leading edge would become visible. Both are module
+constants, not settings.
+
+The two projections have different bases and must stay that way: scene nodes are
+projected from the anchor (what is drawn now), HTML layers from the last settled
+Lit frame (their content is positioned in percentages of that view). They land on
+the same current view, which is what keeps the #451 contract — a marker within
+one CSS pixel of its place in the scene — true on every frame of the gesture.
+
+Neither the attribute nor the style is written when the string is unchanged: an
+idle frame must leave the DOM byte-identical, or the settled raster shifts by a
+few colour levels and golden frames flap. `commitHouseplanViewport` still ends
+the gesture the same way — transforms removed, final `viewBox` forced in.
+
 ## The initial bundle carries English and Russian whole (#400, 2026-08-31)
 
 `en` and `ru` are synchronous dictionaries in the initial chunk; `de` and `fr`

docs/CHANGELOG.md

@@ -8,6 +8,13 @@
   are no longer printed
   ([#530](https://github.com/Matysh/houseplan-card/issues/530)).
 
+- Dragging the plan around is smooth again. Every frame of the gesture used to
+  redraw the entire plan from scratch instead of simply moving what was already
+  drawn, which on a high-refresh screen in Firefox turned panning into a slide
+  show. The plan now slides with the cursor and refreshes its content a few
+  times a second; the picture you end up with is exactly the same
+  ([#531](https://github.com/Matysh/houseplan-card/issues/531)).
+
 ## v1.74.0-beta.2 — 2026-09-11
 
 - A plan left over from the era of “rooms without walls” can be edited again.

docs/CHANGELOG.ru.md

@@ -14,6 +14,13 @@
   внутри комнаты, больше не печатаются
   ([#530](https://github.com/Matysh/houseplan-card/issues/530)).
 
+- Перетаскивание плана снова плавное. Каждый кадр жеста заново рисовал весь
+  план целиком вместо того, чтобы сдвинуть уже нарисованное, — на экране с
+  высокой частотой в Firefox панорама превращалась в слайд-шоу. Теперь план
+  едет за курсором, а содержимое подтягивается несколько раз в секунду; итоговая
+  картинка ровно та же
+  ([#531](https://github.com/Matysh/houseplan-card/issues/531)).
+
 ## v1.74.0-beta.2 — 2026-09-11
 
 - План, доставшийся от времён «комнат без стен», снова редактируется. Такой план