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/commands.rs

@@ -48,6 +48,18 @@ impl OpenCADStudio {
                 return iced::exit();
             }
 
+            // ── Frame-budget HUD (Phase 5.3) ───────────────────────────────
+            // Toggle the per-rebuild wire-tessellation readout overlay.
+            "PERF" => {
+                self.perf_hud = !self.perf_hud;
+                self.command_line.push_info(if self.perf_hud {
+                    "PERF HUD on — shows last wire re-tessellation cost"
+                } else {
+                    "PERF HUD off"
+                });
+                return Task::none();
+            }
+
             // ── Background color ───────────────────────────────────────────
             // Usage:  BACKGROUND <r> <g> <b>   (0–255 each)
             //         BACKGROUND RESET          (restore default)