perf(scene): skip re-tessellation on hover of selected entity

Phase 3.1 (partial). set_hover_highlight bumped the geometry epoch on
every hover-target change, forcing a full model re-tessellation. The
hover handle is folded into the tessellation highlight set as
selected ∪ {hover}, so hovering an already-selected entity contributes
nothing — the effective set and every resulting WireModel are identical.
Guard the bump on the effective highlight contribution actually changing;
hovering over or between selected entities is now free. The field is
still updated so hit-test / UI state stays current.

The full camera/selection-from-tessellation cache split (3.1 / 2.2) is
left open: highlight colour is baked into WireModel.color across several
tessellation sites, so decoupling it needs running-app verification.

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

ROADMAP.md

@@ -162,6 +162,14 @@ geometry is camera-invariant.
   changed),
 - `frame_visible[handle] → bool` (recomputed per `camera_generation`).
 
+**Partial (landed):** `set_hover_highlight` no longer bumps the geometry
+epoch (a full re-tessellation) when the hovered entity is already selected —
+the effective highlight set `selected ∪ {hover}` is then unchanged, so the
+tessellation output is identical. Hovering over / between selected entities
+is now free. The full camera/selection-from-tessellation split is still open
+and needs running-app verification (highlight colour is baked into
+`WireModel.color` across several tessellation sites).
+
 ### 3.2 Persistent GPU buffer pool — diff upload
 
 Today every wire GPU buffer is re-uploaded when