fix: address review findings from PR #402 MEP systems

- core: fix layer violation in level-height.ts — extract sceneRegistry
  import, replace with optional WallBaseYResolver callback so core stays
  pure (no Three.js mesh state); viewer callers pass resolver, headless
  callers (MCP/tests) get deterministic node-data-only result

- core: generalise port-connectivity service from duct-only to all
  distribution families — match partners by distributionRole ('run' →
  endpoint stretch, 'fitting' → rigid follow) instead of hard-coded
  duct-segment/duct-fitting type names; add system-compat guard so
  cross-system ports (e.g. supply duct vs waste pipe) don't fuse

- editor: fix port-snap rotation bug in move tool — pass preview node
  at live rotation into resolvePortSnap so own-port positions reflect
  any mid-drag R/T rotation before computing the snap delta

- editor: wire pipe-trap into UI — add to StructureTool union,
  MepToolKind, MEP_ITEMS Build-tab tile, and structure-tools action menu

- test: add port-connectivity-pipe.test.ts — 2 tests covering
  pipe-fitting → pipe-segment endpoint drag and cross-system isolation

- test: fix stale pipe-auto-fitting.test.ts wye expectation — author
  deliberately chose square sanitary-tee for DWV side-taps (documented
  in PR description and PipeFittingNode schema); update the one test
  that still expected wye to match the implemented behaviour

- nit: fix optional-chain biome warning in validate-dwv.ts

apps/editor/components/build-tab.tsx

@@ -44,6 +44,7 @@ type MepToolKind =
   | 'liquid-line'
   | 'pipe-segment'
   | 'pipe-fitting'
+  | 'pipe-trap'
 
 type BuildType = {
   /** Selection id — equals `kind` for tool types, `'painting'` for paint mode, `'mep'` for the MEP group. */
@@ -98,6 +99,7 @@ const MEP_ITEMS: MepItem[] = [
   { id: 'lineset', label: 'Lineset', iconSrc: '/icons/lineset.png', kind: 'lineset' },
   { id: 'liquid-line', label: 'Liquid Line', iconSrc: '/icons/lineset.png', kind: 'liquid-line' },
   { id: 'pipe-segment', label: 'DWV Pipe', iconSrc: '/icons/dwv-pipes.png', kind: 'pipe-segment' },
+  { id: 'pipe-trap', label: 'Trap', iconSrc: '/icons/dwv-pipes.png', kind: 'pipe-trap' },
 ]
 
 /**