Stream deferred visual nodes in camera-distance order (nearest first)

The progressive load previously streamed the deferred visual nodes (3d model
instances + terrain shapes/lines) in file order, so distant scenery could load
before the player's surroundings. This builds them nearest-camera first instead.

The visual pass now runs in two steps. Enumeration replays the twin and captures
each visual node verbatim (its resolved tokens as text -- numbers round-trip
losslessly through cParser) together with the transform/group context it was read
under and, for models, its transformed world position. Once the replay is
exhausted the records are sorted by squared distance to the camera (terrain shapes
first so the ground appears before the props on it), then built a budgeted slice
per frame through the normal node path with the captured transform and group
restored -- so placement, grouping and the per-cell instance buckets come out
identical to an in-order load.

Two supporting fixes make out-of-order/late insertion correct:
- a cell/section whose geometry was already baked (the renderer finalised it
  before a deferred node arrived) now appends the new shape/lines straight into
  its live geometry bank instead of merging into vertex-freed geometry, which
  would silently drop it; create_geometry() remembers the bank for every cell.
- events that bind to visual model instances (lights/animation/texture/visible)
  are deferred from InitEvents() to a new InitInstanceEvents() run after the
  visual nodes are built, so their target models exist when they initialise.

Verified on td.scn: playable ~2s, 540 deferred nodes enumerated and built
nearest-first ~0.5s later, no duplicate instances.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

scene/scenenodegroups.h

@@ -40,6 +40,13 @@ public:
     // returns current active group, or null_handle if group stack is empty
     group_handle
         handle() const;
+    // makes Group the active group for subsequent inserts, without creating a new group.
+    // used by deferred (camera-ordered) visual loading to re-attach a node to the group it
+    // was originally read under during the enumeration pass. must be paired with pop_active().
+    void
+        push_active( scene::group_handle const Group ) { m_activegroup.push( Group ); }
+    void
+        pop_active() { if( false == m_activegroup.empty() ) { m_activegroup.pop(); } }
     // places provided node in specified group
     void
         insert( scene::group_handle const Group, scene::basic_node *Node );