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/liquid-line/options.ts

@@ -0,0 +1,21 @@
+import { create } from 'zustand'
+
+/**
+ * Shared draw-time options for the liquid-line tool. Lives in the nodes
+ * package so both the tool (which reads + key-toggles it) and the app's MEP
+ * panel (which renders the toggle button) can bind to the same state.
+ *
+ * `follow` arms "trace a lineset": while on, clicking an existing lineset
+ * lays a liquid line beside it along the same path instead of free-drawing.
+ */
+type LiquidLineToolOptions = {
+  follow: boolean
+  setFollow: (value: boolean) => void
+  toggleFollow: () => void
+}
+
+export const useLiquidLineToolOptions = create<LiquidLineToolOptions>((set) => ({
+  follow: false,
+  setFollow: (value) => set({ follow: value }),
+  toggleFollow: () => set((s) => ({ follow: !s.follow })),
+}))