Canvas view changes trigger a full engine round-trip per frame during pan/zoom #707

Problem

Every canvas view change runs a full engine round-trip. Canvas.tsx calls onViewBoxChange on:

• every wheel event,
• every momentum-animation requestAnimationFrame frame, and
• every pinch update.

Editor.queueViewUpdate then awaits, per event:

• engine.applyPatch (upsertView),
• engine.serializeProtobuf,
• a full serializeJson, and
• a projectFromJson re-parse of the entire project.

So a single pan or pinch gesture fires this entire serialize/re-parse pipeline on every animation frame.

Why it matters

On large models this makes panning/zooming jankier than necessary and burns WASM CPU. View changes are purely presentational, so doing full project serialization + re-parse per frame is wasted work in the interactive hot path -- a developer-experience and performance concern.

Components affected

• src/diagram (Canvas.tsx, Editor.tsx -- queueViewUpdate)
• src/engine (WASM applyPatch / serializeProtobuf / serializeJson / projectFromJson)

Context

Identified during the frontend-react-audit branch review (June 2026). As of that branch, these view-only updates no longer pollute the undo history (the worst user-visible symptom has been fixed), but the per-frame persistence work remains.

Suggested fix

Debounce/coalesce view persistence: only push the final viewBox to the engine on gesture end or after ~250ms idle, while keeping the optimistic local view state immediate so the canvas stays responsive.