fix: share block definition geometry across viewport slots

Zooming a paper layout out until several viewports fit on screen ran the
card out of memory. The log correlates exactly: every error cluster
follows a `nav-prepare … space=Paper` with `viewports=3`, `4` or `5`,
where earlier sessions had reported `viewports=2` and stayed inside the
budget.

Each viewport gets its own `Pipeline` slot, and `block_geometry` was a
slot field — so every slot uploaded its own copy of the same definition
segments. That is the single largest upload this renderer makes: 1 101
definitions, 1 040 267 segments, 59.5 MiB. Three of them land back to
back in one burst in the log, each bracketed by its own full arena
rebuild.

The cache key is `(source_id, mesh_edge, colour, base translation)` —
nothing a viewport contributes. Two viewports drawing the same definition
draw the same segments, so the cache belongs beside `wire_buffer_cache`
on the `MultiPipeline`, which already shares the finished `BlockWireGpu`
batches across slots for the same reason.

That existing sharing also settles the one question this raises: bind
groups built against one slot's layout are already handed to other slots
today, so nothing new is being asked of wgpu here. Both GPU regression
tests still pass on a real adapter with validation scopes armed.

This does not address the wire arena, which stays per slot and rebuilds
52 MiB of instances each time a slot changes hands — `arena-base
ok=false` on every single build in the session, never `ok=true`.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/scene/view/render.rs

@@ -860,6 +860,7 @@ impl shader::Primitive for Primitive {
                                 queue,
                                 &instanced,
                                 &draw_depths,
+                                &mut pipeline.block_geometry,
                             ),
                         );
                         if _patched {
@@ -936,7 +937,13 @@ impl shader::Primitive for Primitive {
                             let errors_before =
                                 crate::scene::pipeline::gpu_errors_seen();
                             let entry =
-                                inner.build_wire_buffers(device, queue, &vp_wires[..], &draw_depths);
+                                inner.build_wire_buffers(
+                                    device,
+                                    queue,
+                                    &vp_wires[..],
+                                    &draw_depths,
+                                    &mut pipeline.block_geometry,
+                                );
                             if let Some(start) = t_upload {
                                 crate::perf_record!(
                                     "[perf] wire-upload {:.1}ms wires={} content_id={}",