fix: address 2nd-round adversarial review of PR #402 fixes

- core: decouple drag-follow from distributionRole — add
  portConnectivityFollow flag to NodeDefinition; pipe-trap opts out
  (portConnectivityFollow: false) so dragging a connected pipe endpoint
  stretches the trap arm instead of yanking the anchored trap fixture

- core: remove the module-level getLevelHeight cache entirely — it was
  keyed only by nodes-object identity, which could return stale heights
  for in-place mutations by pure/headless callers. The function is now
  fully pure and deterministic; viewer hot path recomputes per frame as
  before (the cache only ever skipped the resolver-free branch)

- test: harden port-connectivity-pipe.test.ts — real DuctSegmentNode
  cross-family isolation case (was waste-vs-vent), new pipe-trap anchor
  case (run drag doesn't move trap; trap drag still stretches run), and
  beforeEach/afterEach registry reset instead of leaky beforeAll

- nit: biome format/import-order on all touched files

packages/core/src/registry/types.ts

@@ -720,6 +720,20 @@ export type NodeDefinition<S extends ZodObject<any>> = {
    * Kinds outside any distribution system leave this unset.
    */
   distributionRole?: DistributionRole
+  /**
+   * When `distributionRole` is `'fitting'`, controls whether this fitting
+   * is dragged as a rigid follower when a connected run endpoint moves.
+   *
+   * - `true` (default for `distributionRole === 'fitting'`): the fitting
+   *   translates rigidly so its mated collar stays on the moved port — the
+   *   right behaviour for in-line fittings (elbows, tees, wyes, crosses).
+   * - `false`: the fitting is anchored in space; moving a connected run
+   *   endpoint stretches the run arm, not the fitting. Use this for
+   *   fixed-position fixtures like `pipe-trap`.
+   *
+   * Has no effect when `distributionRole` is not `'fitting'`.
+   */
+  portConnectivityFollow?: boolean
 
   defaults: () => Omit<z.infer<S>, 'id' | 'type'>
   migrate?: Record<number, (old: unknown) => unknown>