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>
