perf(blocks): keep repeated block copies as one stub wire each

Every further reference to an already expanded block (a translated copy
of the same definition, at the top level or nested) was materialised as
a full copy of every wire, although the GPU already draws those copies
from one shared source with a per-copy offset. Such a copy is now a
single stub wire holding the shared prototype and its offset (stubs nest
inside prototypes, so nested repeats collapse too).

- The block renderer expands stubs into instance references while it
  groups by source, so drawing is unchanged.
- Interaction queries expand only the stubs near the query, so snapping,
  picking, window and crossing selection see each copy's geometry.
- Plot, boundary detection, viewport projection and entity_wires() get
  fully expanded wires; count and reference-edit recolouring expand the
  set while they are active.
- OCS_NO_BLOCK_STUBS turns it off.

A drawing with 480 references to a block of 100 nested 50-line blocks
(2.4M lines) now opens with 11k wires instead of 2.4M: peak memory
5.4 GB -> 0.37 GB, wire assembly 2.4 s -> 36 ms, interaction index
3.7 s -> 23 ms. The viewport, a plotted PDF, snaps and selections are
identical with and without stubs, and on five real drawings the view
renders the same.

Refs #1698, #1542

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/scene/text/complex_lt.rs

@@ -386,6 +386,7 @@ pub fn apply_along(
             fill_is_3d: false,
             fill_is_2d_solid: false,
             render_instance: None,
+            stub: None,
             pick_tris: Vec::new(),
             pick_tris_low: Vec::new(),
             dash_from_start: false,