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/floorplan.ts

@@ -0,0 +1,58 @@
+import type { FloorplanGeometry, GeometryContext } from '@pascal-app/core'
+import { INCHES_TO_METERS } from '../duct-segment/geometry'
+import { getPipeFittingPorts } from './ports'
+import type { PipeFittingNode } from './schema'
+
+const WASTE_COLOR = '#57534e'
+const VENT_COLOR = '#78716c'
+
+/**
+ * Floor-plan symbol for a DWV fitting: one line per collar from the
+ * junction out (a wye's 45° branch reads at its true plan angle), plus
+ * a hub circle. Vertical collars (stack connections) collapse onto the
+ * hub, which is how they should read from above.
+ */
+export function buildPipeFittingFloorplan(
+  node: PipeFittingNode,
+  ctx: GeometryContext,
+): FloorplanGeometry | null {
+  const [cx, , cz] = node.position
+  const view = ctx.viewState
+  const palette = view?.palette
+  const showSelectedChrome = (view?.selected || view?.highlighted) ?? false
+  const stroke =
+    showSelectedChrome && palette
+      ? palette.selectedStroke
+      : node.system === 'vent'
+        ? VENT_COLOR
+        : WASTE_COLOR
+
+  const children: FloorplanGeometry[] = []
+  for (const port of getPipeFittingPorts(node)) {
+    const px = port.position[0]
+    const pz = port.position[2]
+    if (Math.hypot(px - cx, pz - cz) < 1e-4) continue
+    children.push({
+      kind: 'line',
+      x1: cx,
+      y1: cz,
+      x2: px,
+      y2: pz,
+      stroke,
+      strokeWidth: port.diameter * INCHES_TO_METERS,
+      strokeLinecap: 'round',
+      opacity: showSelectedChrome ? 0.95 : 0.85,
+    })
+  }
+  children.push({
+    kind: 'circle',
+    cx,
+    cy: cz,
+    r: (node.diameter * INCHES_TO_METERS) / 2 + 0.012,
+    fill: stroke,
+    opacity: 0.95,
+  })
+  if (showSelectedChrome) children.push({ kind: 'move-handle', point: [cx, cz] })
+
+  return { kind: 'group', children }
+}