feat(worldgraph): ADR-139 WorldGraph environmental digital twin (#843)

New crate wifi-densepose-worldgraph:
- model.rs: WorldNode (10 kinds) + WorldEdge (7 relations) as serde enums (no
  trait objects → deterministic RVF persistence); WorldId, EnuPoint,
  ZoneBoundsEnu (with point-in-bounds), SemanticProvenance (house-rule tuple)
- graph.rs: WorldGraph over petgraph StableDiGraph; upsert/add_edge/neighbors,
  room_for_area (HomeCore area_id linkage), observed_by/contents_of queries,
  add_semantic_state (append-with-provenance DerivedFrom), add_contradiction
  (both beliefs retained), apply_privacy_mode → PrivacyRollup, JSON persistence
- 7 tests (upsert/replace, linkage, unknown-endpoint, location, provenance+
  contradiction, privacy rollup, deterministic JSON round-trip)
- workspace 0 errors

Co-Authored-By: claude-flow <ruv@ruv.net>