fix(viewport): per-tile wire cache for Model layout

Tiled Model panes were all reading from the shared `entity_wires_arc`,
whose tessellation runs `view_world_aabb` / `world_per_pixel` against
the live `Scene::camera`. Zooming the active tile re-built that arc
with the active camera's LOD / frustum cull, then every other tile
rendered from the same arc — content in the inactive panes flickered
between full and LOD-stub tessellations as you zoomed.

- `wires_for_block` now delegates to `wires_for_block_culled`, which
  takes `view_aabb` + `wpp` explicitly. The original caller passes
  the live-camera values; tile rendering passes its own.
- New `model_tile_wires_arc` builds a per-tile view AABB + wpp from
  the tile's camera + pixel rect and caches the result. Cache is
  keyed by tile index; the value holds `(geometry_epoch,
  camera_state_hash)`, so a stale hit (camera moved, doc changed)
  still misses and rebuilds.
- `ViewportInstance` gains `tile_idx` so `viewport_data_for` can route
  Model tiles into the per-tile cache without disturbing the paper-
  layout content-viewport path (it still uses `model_wires_for_viewport_arc`).

src/scene/render.rs

@@ -580,7 +580,18 @@ impl Scene {
         let us = (visible_w / full.width).clamp(0.0, 1.0);
         let vs = (visible_h / full.height).clamp(0.0, 1.0);
 
-        let base_arc = if inst.handle == acadrust::Handle::NULL {
+        // Model tiles each get their own tessellation cache keyed off the
+        // tile's camera — culling/LOD depend on per-tile zoom, not the
+        // active tile's. Paper-space content viewports already had a
+        // per-viewport cache.
+        let base_arc = if let Some(tile_idx) = inst.tile_idx {
+            let aspect = if full.height > 0.0 {
+                full.width / full.height
+            } else {
+                1.0
+            };
+            self.model_tile_wires_arc(tile_idx, &inst.camera, aspect, full.height)
+        } else if inst.handle == acadrust::Handle::NULL {
             self.entity_wires_arc()
         } else {
             self.model_wires_for_viewport_arc(inst.handle)