Add HostingService with cycle/depth/kind validation (Phase 1, 2/6)

First service in `core/src/services/`: pure, no React or R3F, takes
SceneApi + node data, returns results.

Exports:
- `canAttach(childId, hostId, scene)` — validates host attachment.
  Rejects self-host, cycles (host's ancestor chain contains child),
  chains past MAX_HOST_DEPTH (6), and host kinds outside the child
  def's `capabilities.hostable.parents` allowlist. Returns a typed
  AttachError discriminated union so callers can render specific
  messages.
- `getSurface(host)` / `getTopSurfaceHeight(host)` — reads
  `def.capabilities.surfaces` from the registry; resolves
  function-valued heights with the node.
- `clampYToHostTop(host, y)` — convenience for placement code.
- `pickHost({ point, candidates, placedKind, hitTest? })` — given
  spatially pre-filtered candidates, returns the first hostable.
  The runtime is responsible for spatial filtering; this function
  stays pure.

MAX_HOST_DEPTH = 6: the explore earlier found today's editor has no
cap on item-on-item nesting. Cap is bounded by hostable depth, not
total tree depth (sites/buildings/levels don't count).

17 tests cover all rejection paths + happy paths + function-valued
surface heights.

Re-exported from `@pascal-app/core` via a new `services/` barrel.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/core/src/index.ts

@@ -65,6 +65,7 @@ export {
 } from './material-library'
 export * from './registry'
 export * from './schema'
+export * from './services'
 export {
   getSceneHistoryPauseDepth,
   pauseSceneHistory,