perf(scene): memoize Face3D wire split across frames

build_primitive ran split_face3d_wires every frame — for each wire a
string-parse + document handle lookup + clone, to separate Face3D wires
from the rest — even when a pan had just reused the tessellation. The
split is content-only, so memoize it by the Model-tile wire content id:
a pan-reuse (same id) reuses the split, and the common no-overlay frame
returns the cached `other` Arc with zero clone. Preview/interim frames
clone once to append. Non-tile paths split inline as before.

Completes the cheap-pan path: a pure pan now reuses the CPU
tessellation, the Face3D split, and the GPU wire upload — only the
view_proj uniform and per-frame scissors recompute.

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

ROADMAP.md

@@ -176,6 +176,13 @@ outright — no re-tessellation, `tess ms` drops to ~0 on the PERF HUD. Zoom
 exactly as before; the cull margin is unchanged, so miss cost is identical
 (no zoom regression).
 
+**Partial (landed): per-frame split.** `build_primitive` ran
+`split_face3d_wires` every frame — an O(N) per-wire handle lookup + clone to
+separate Face3D wires — even on a pan that reused the tessellation. It's now
+memoized by the tile's wire content id, and the non-overlay frame reuses the
+`other` Arc with no clone at all. So a pan reuses tessellation, the split, and
+the GPU upload; only the uniform + scissors update per frame.
+
 **Partial (landed): hover.** `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