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/services/level-height.ts

@@ -7,40 +7,18 @@ export const DEFAULT_LEVEL_HEIGHT = 2.5
  * Optional resolver for a wall's rendered base Y (mesh elevation).
  *
  * `packages/core` is pure domain logic and must not read viewer/Three.js
- * state (see AGENTS.md "Layer Boundaries"). Callers that legitimately have
+ * state (see AGENTS.md “Layer Boundaries”). Callers that legitimately have
  * registry access (viewer systems, node tools) may pass a resolver so the
  * mesh elevation is factored in; pure/headless callers (MCP, tests, server)
  * omit it and get a deterministic result from serialized node data alone.
  */
 export type WallBaseYResolver = (wallId: AnyNodeId) => number | undefined
 
-// Cache: levelId → computed height. Invalidated when the nodes reference changes.
-// Zustand produces a new `nodes` object on every mutation, so reference equality
-// is a zero-cost way to detect stale data without any subscription overhead.
-//
-// NOTE: the cache is keyed only by the `nodes` reference, so it is only safe to
-// reuse across calls that pass the same resolver semantics. When a resolver is
-// supplied (viewer path), mesh Y can change without a `nodes` reference change,
-// so the viewer caller must not rely on the cache for live values — it passes a
-// resolver and we recompute. We therefore only cache resolver-free (pure)
-// results, which are a deterministic function of `nodes`.
-const heightCache = new Map<string, number>()
-let lastNodesRef: object | null = null
-
 export function getLevelHeight(
   levelId: string,
   nodes: Record<AnyNodeId, AnyNode>,
   resolveWallBaseY?: WallBaseYResolver,
 ): number {
-  if (nodes !== lastNodesRef) {
-    heightCache.clear()
-    lastNodesRef = nodes
-  }
-
-  // Only the pure (resolver-free) computation is cacheable: with a resolver the
-  // result can depend on live mesh state that the `nodes` reference doesn't track.
-  if (!resolveWallBaseY && heightCache.has(levelId)) return heightCache.get(levelId)!
-
   const level = nodes[levelId as LevelNode['id']] as LevelNode | undefined
   if (!level) return DEFAULT_LEVEL_HEIGHT
 
@@ -60,7 +38,5 @@ export function getLevelHeight(
     }
   }
 
-  const height = maxTop > 0 ? maxTop : DEFAULT_LEVEL_HEIGHT
-  if (!resolveWallBaseY) heightCache.set(levelId, height)
-  return height
+  return maxTop > 0 ? maxTop : DEFAULT_LEVEL_HEIGHT
 }

packages/core/src/services/port-connectivity.ts

@@ -112,8 +112,12 @@ export function analyzePortConnectivity(
     // Generalised across every distribution family (HVAC duct + DWV pipe):
     // `run` partners stretch an endpoint, `fitting` partners follow rigidly.
     // Terminals/equipment mount to surfaces and are intentionally NOT dragged.
+    // Fittings that declare `portConnectivityFollow: false` are anchored
+    // fixtures (e.g. pipe-trap) — moving a connected run stretches the arm.
     const otherRole = roleOf(other)
     if (otherRole !== 'run' && otherRole !== 'fitting') continue
+    const otherDef = nodeRegistry.get(other.type)
+    if (otherRole === 'fitting' && otherDef?.portConnectivityFollow === false) continue
     const otherPorts = portsOf(other)
     if (!otherPorts) continue