feat: HVAC ductwork + DWV plumbing systems (#402)

Adds two new MEP node families (HVAC ductwork, DWV plumbing) built on a shared port-connectivity model. Co-authored by @sudhir9297.

packages/nodes/src/shared/path-point-affordance.ts

@@ -0,0 +1,68 @@
+import {
+  type AnyNodeId,
+  type FloorplanAffordance,
+  type FloorplanAffordanceSession,
+  useScene,
+} from '@pascal-app/core'
+import { snapPointToGrid, type WallPlanPoint } from '@pascal-app/editor'
+
+/**
+ * Shared "drag a path point" floor-plan affordance for polyline
+ * distribution kinds (duct-segment / pipe-segment / lineset). It is the
+ * 2D counterpart of their 3D `affordanceTools.selection` handles: one
+ * draggable handle per path vertex, moved freely on the plan (XZ) with
+ * grid snap (Shift bypasses). The vertex's Y (elevation / slope) is held
+ * fixed — plan editing never changes height.
+ *
+ * Wired via `def.floorplanAffordances['move-path-point']`; the floor-plan
+ * builders emit `endpoint-handle` primitives carrying `{ pointIndex }` so
+ * the dispatcher routes pointer-downs here.
+ */
+export type PathPointPayload = { pointIndex: number }
+
+type PathShape = { path: ReadonlyArray<readonly [number, number, number]> }
+
+export function createPathPointMoveAffordance<N extends PathShape & { id: AnyNodeId }>(
+  kind: string,
+): FloorplanAffordance<N> {
+  const inert: FloorplanAffordanceSession = {
+    affectedIds: [],
+    apply() {},
+    canCommit() {
+      return false
+    },
+  }
+  return {
+    start({ node, payload }): FloorplanAffordanceSession {
+      const { pointIndex } = payload as PathPointPayload
+      const initialPath = node.path.map((p) => [...p] as [number, number, number])
+      const target = initialPath[pointIndex]
+      if (!target) return { ...inert, affectedIds: [node.id] }
+      // Hold the dragged vertex's elevation — the plan move only shifts XZ.
+      const y = target[1]
+
+      return {
+        affectedIds: [node.id],
+        apply({ planPoint, modifiers }) {
+          // Plan coords map x→world X, y→world Z.
+          const raw: WallPlanPoint = [planPoint[0], planPoint[1]]
+          const [sx, sz] = modifiers.shiftKey ? raw : snapPointToGrid(raw)
+          const nextPath = initialPath.map((p, i) =>
+            i === pointIndex ? ([sx, y, sz] as [number, number, number]) : p,
+          )
+          useScene
+            .getState()
+            .updateNodes([{ id: node.id, data: { path: nextPath } as Partial<unknown> as never }])
+        },
+        canCommit() {
+          const final = useScene.getState().nodes[node.id] as N | undefined
+          return (
+            !!final &&
+            (final as unknown as { type: string }).type === kind &&
+            final.path.length >= 2
+          )
+        },
+      }
+    },
+  }
+}