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/pipe-fitting/selection.tsx

@@ -0,0 +1,43 @@
+'use client'
+
+import { type AnyNodeId, useScene } from '@pascal-app/core'
+import { useViewer } from '@pascal-app/viewer'
+import { useEffect } from 'react'
+import { cycleRotationAxis } from '../shared/fitting-rotation'
+
+/**
+ * Selection-time rotation support for placed pipe fittings — mirrors
+ * the duct-fitting affordance, mounted by the editor's
+ * SelectionAffordanceManager (`def.affordanceTools.selection`). R/T
+ * rotation lives in `def.keyboardActions`; this contributes the piece
+ * that hook can't: **Alt cycles the active rotation axis** while a
+ * single fitting is selected. The axis lives on `useEditor.rotationAxis`,
+ * which the floating action menu reads to show the axis pill — so this
+ * component renders nothing.
+ */
+const PipeFittingSelectionAffordance = () => {
+  const selectedIds = useViewer((s) => s.selection.selectedIds)
+  const hasSelectedFitting = useScene((s) => {
+    if (selectedIds.length !== 1) return false
+    return s.nodes[selectedIds[0] as AnyNodeId]?.type === 'pipe-fitting'
+  })
+
+  useEffect(() => {
+    if (!hasSelectedFitting) return
+    const onKeyDown = (e: KeyboardEvent) => {
+      if (e.key !== 'Alt' || e.repeat) return
+      const tag = (e.target as HTMLElement | null)?.tagName
+      if (tag === 'INPUT' || tag === 'TEXTAREA') return
+      e.preventDefault()
+      cycleRotationAxis()
+    }
+    // Bubble phase — when the placement tool is active its capture-phase
+    // handler stops propagation, so the two never double-cycle.
+    window.addEventListener('keydown', onKeyDown)
+    return () => window.removeEventListener('keydown', onKeyDown)
+  }, [hasSelectedFitting])
+
+  return null
+}
+
+export default PipeFittingSelectionAffordance