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The first half of the eventual full DCEL — vertex identity. Every parcel polygon vertex now resolves to a stable VertexId via a spatial-hash lookup at EPS_GEOM resolution. Coincident positions across two parcels resolve to the same VertexId, so adjacent parcels share one physical vertex. ParcelSet::move_vertex(vid, new_pos) updates the registry's position AND writes through to every parcel's polygon at the recorded index. Adjacent parcels' shared boundaries can never drift apart — they are the same vertex, mutated once. The deform pipeline now propose-then-apply: deform_parcel_after_road_move returns proposed (VertexId, new_pos) moves rather than mutating in place. The outer loop validates each parcel's hypothetical post-move polygon, then applies all proposed moves via move_vertex. Conflicting proposals on the same vertex are last-one-wins, but in practice the deform parameterization makes all referrers agree by construction. New regression test: shared_vertex_no_drift_under_repeated_edits. 50 small random node moves plus an inverse; asserts every shared boundary vertex is bit-for-bit identical across every parcel that references it. Edge identity (parcel-layer half-edges) is the next milestone — it enables split/merge ergonomics. Vertex identity alone is enough for the no-drift contract this session was scoped to deliver. Journal §11 session 4 entry adds D17 (registry), D18 (write-through), D19 (propose-then-apply), and the milestone-0.5 queue. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>