v0.16.0 — magnetic docking

[Coldbari]

Connect by touching instead of by drawing.

Drag a symbol — in from the palette, or one already on the sheet — so one of its
connection points comes within ~18 screen px of another symbol's, and a ring
marks the point it has caught while every connection point lights up. Let go and
the symbol clicks into place, the two points landing on exactly the same spot,
with the line already drawn.

Because a line stores ports and not coordinates, pulling the pair apart
afterwards stretches the pipe instead of breaking it.

Details that make it behave

• Line class comes from the ports, not the toolbar. A controller docking
  onto a valve's signal boss gets signal.electric even with a process class
  selected. Incompatible pairings never dock.
• Reach is judged in screen px, so it feels the same at any zoom — clamped
  in sheet space so an extreme zoom can neither make it unhittable nor grabby
  enough to swallow neighbouring symbols.
• An already-joined pair won't dock again, so nudging a docked symbol can't
  stack a second identical line on the first.
• One undo step for the symbol and the line it docked onto, whether it came
  from the palette (addBatch) or a drag across the sheet (dockNode).
• The hint ring lives inside .joint-layers, the group carrying the pan/zoom
  transform, so it sits on the sheet rather than floating over the viewport.

One change on top of the original work

Added a prefers-reduced-motion guard so the ring stops pulsing for users who
ask for less motion — it still marks the spot. The rest of this stylesheet
already honours that, so the new animation was the odd one out.

Verification

• npm run build — clean
• npm test — 724 passing (10 new: findDock / dockEdge / dockRadius,
  covering rotation, zoom clamping, closest-candidate selection, incompatible
  ports, and the already-joined guard)
• npx playwright test — 62 passing, 1 skipped (3 new: a palette drop that
  docks, a drop clear of everything that simply lands, and a drag that docks
  then stretches when pulled away)

🤖 Generated with Claude Code