fix(paper): reset a reused viewport GPU slot so the survivor keeps its content

Pipeline slots are addressed by list index, but the renderer drops
off-canvas viewports from the list, so a slot can be reused by a
different viewport across frames — e.g. when the first paper viewport
scrolls off the canvas the second slides into its slot and inherits its
stale, differently frustum-culled GPU buffers. The text upload is gated
on a geometry-epoch change (unaffected by the index shift), so the
survivor rendered the gone viewport's text and its own vanished.

Give each viewport a stable identity, remember which one last used a
slot, and reset all cache keys when the occupant changes so wires, text,
hatches, meshes and Face3D re-upload for the new viewport. No cost in the
steady state — the reset fires only when a viewport enters/leaves view.

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

src/scene/pipeline/mod.rs

@@ -228,6 +228,12 @@ pub struct Pipeline {
     /// `render` skips the (per-pixel, GPU-dominating) hatch pass this frame. The
     /// scene-render cache holds the full-quality frame once the view settles.
     pub skip_hatch_frame: bool,
+    /// Stable identity of the viewport that last used this (index-addressed)
+    /// pipeline slot. The renderer's viewport list drops off-canvas viewports,
+    /// so a slot can be reused by a *different* viewport across frames; when the
+    /// occupant changes, all cache keys above belong to the previous one and are
+    /// reset so every buffer re-uploads. `u64::MAX` = never used.
+    pub slot_id: u64,
 }
 
 impl Pipeline {
@@ -1226,6 +1232,7 @@ impl Pipeline {
             render_sig: u64::MAX,
             skip_geometry: false,
             skip_hatch_frame: false,
+            slot_id: u64::MAX,
         }
     }