feat(scene): PERF frame-budget HUD for wire re-tessellation cost

Phase 5.3 (CPU tessellation slice). Add a `PERF` command that toggles an
overlay on the active viewport showing the cost of the most recent wire
re-tessellation: ms, wire count, geometry epoch. Anchored top-left via
position_canvas_overlay.

Timing is captured at the wire-cache miss paths (model_tile_wires_arc,
paper_sheet_wires_arc) and stored in Cells on Scene, read directly by the
UI view (Scene is single-threaded / Rc-based, so build_primitive and view
share a thread). Reads ~0 ms on a warm cache, so it isolates the exact
work a miss costs — the slice 2.2 / 3.1 / 3.3 each move, making them
PR-to-PR comparable.

GPU upload/draw/GPU-wait spans (need wgpu timestamp queries) remain open.

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

src/app/mod.rs

@@ -186,6 +186,9 @@ pub(super) struct OpenCADStudio {
     /// Selection cycling: clicking where objects overlap opens a list box
     /// to pick which one; the pick is added to the current selection.
     selection_cycling: bool,
+    /// Frame-budget HUD (Phase 5.3): overlays the last wire re-tessellation
+    /// cost on the active viewport. Toggled by the `PERF` command.
+    perf_hud: bool,
     /// When set, the cycling list box is open: (canvas point, candidates).
     cycle_candidates: Option<(iced::Point, Vec<acadrust::Handle>)>,
     /// Which status-bar pills the user has chosen to show (persisted).
@@ -1254,6 +1257,7 @@ impl OpenCADStudio {
             clean_screen: false,
             quick_properties: false,
             selection_cycling: false,
+            perf_hud: false,
             cycle_candidates: None,
             pre_cmd_tangent: None,
             ortho_mode: false,