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>

ROADMAP.md

@@ -261,10 +261,17 @@ Opened "x.dwg" — 84321 entities — parse 1.2s, purge 80ms, caches 340ms, xref
 
 Regressions are visible immediately.
 
-### 5.3 Frame-budget HUD
+### 5.3 Frame-budget HUD ✅ DONE (CPU tess slice)
 
-Add a CLI `PERF` toggle (or F12) that shows per-frame breakdown: tess ms,
+A CLI `PERF` toggle overlays the cost of the most recent wire
-upload ms, draw ms, GPU wait ms. Makes PR-to-PR comparison trivial.
+re-tessellation (ms + wire count + geometry epoch) on the active viewport,
+anchored top-left. Reads ~0 ms while the wire cache is warm (idle
+pan/zoom), so it isolates exactly the work a cache miss costs — the slice
+every render-path change (2.2 / 3.1 / 3.3) moves. Timed at the miss paths
+in `model_tile_wires_arc` / `paper_sheet_wires_arc` and stored on `Scene`.
+
+Still open: GPU-side `upload` / `draw` / `GPU-wait` spans need wgpu
+timestamp queries; the CPU tessellation slice covers the current hot path.
 
 ---
 

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)

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,

src/app/view.rs

@@ -1014,6 +1014,48 @@ impl OpenCADStudio {
             }
         }
 
+        // Frame-budget HUD (Phase 5.3): toggle with the PERF command. Shows
+        // the cost of the most recent wire re-tessellation — the work avoided
+        // by a warm wire cache — so render-path changes can be compared
+        // PR-to-PR. Reads ~0 ms while panning/zooming on a hit cache.
+        if self.perf_hud && !tab.is_start {
+            let s = &tab.scene;
+            let label = format!(
+                "tess {:.1} ms · {} wires · epoch {}",
+                s.last_tess_ms.get(),
+                s.last_tess_wires.get(),
+                s.geometry_epoch,
+            );
+            let panel = container(text(label).size(12).color(Color {
+                r: 0.6,
+                g: 1.0,
+                b: 0.6,
+                a: 1.0,
+            }))
+            .padding(6)
+            .style(|_: &Theme| container::Style {
+                background: Some(Background::Color(Color {
+                    r: 0.08,
+                    g: 0.08,
+                    b: 0.08,
+                    a: 0.85,
+                })),
+                border: Border {
+                    color: Color {
+                        r: 0.35,
+                        g: 0.35,
+                        b: 0.35,
+                        a: 1.0,
+                    },
+                    width: 1.0,
+                    radius: 3.0.into(),
+                },
+                ..Default::default()
+            });
+            viewport_stack = viewport_stack
+                .push(position_canvas_overlay(iced::Point::new(12.0, 40.0), panel.into()));
+        }
+
         // Selection-cycling list box: pick among overlapping objects.
         if let Some((pt, cands)) = &self.cycle_candidates {
             if !tab.is_start {

src/scene/mod.rs

@@ -712,6 +712,13 @@ pub struct Scene {
     /// without depending on the shader widget's internal `Program::State`
     /// (which can miss events under overlapping overlays).
     pub viewcube_hover: std::cell::Cell<Option<usize>>,
+    /// Wall time (ms) of the most recent wire re-tessellation — the work done
+    /// on a wire-cache miss in `model_tile_wires_arc` / `paper_sheet_wires_arc`.
+    /// Stays at the last value while the cache is hit (idle pan/zoom on a warm
+    /// cache reads ~0). Surfaced by the frame-budget HUD (Phase 5.3).
+    pub(crate) last_tess_ms: std::cell::Cell<f32>,
+    /// Wire count produced by that most recent re-tessellation.
+    pub(crate) last_tess_wires: std::cell::Cell<usize>,
 }
 
 impl Scene {
@@ -769,6 +776,8 @@ impl Scene {
             last_render_aspect: std::cell::Cell::new(16.0 / 9.0),
             last_world_per_pixel: std::cell::Cell::new(0.0),
             viewcube_hover: std::cell::Cell::new(None),
+            last_tess_ms: std::cell::Cell::new(0.0),
+            last_tess_wires: std::cell::Cell::new(0),
         }
     }
 
@@ -1570,7 +1579,10 @@ impl Scene {
             None
         };
         let block = self.model_space_block_handle();
+        let t_tess = std::time::Instant::now();
         let arc = Arc::new(self.wires_for_block_culled(block, view_aabb, wpp, None, None));
+        self.last_tess_ms.set(t_tess.elapsed().as_secs_f32() * 1000.0);
+        self.last_tess_wires.set(arc.len());
         self.model_tile_wire_cache
             .borrow_mut()
             .insert(tile_idx, (key, Arc::clone(&arc)));
@@ -1590,7 +1602,10 @@ impl Scene {
             }
         }
         let layout_block = self.current_layout_block_handle();
+        let t_tess = std::time::Instant::now();
         let mut wires = self.wires_for_block(layout_block);
+        self.last_tess_ms.set(t_tess.elapsed().as_secs_f32() * 1000.0);
+        self.last_tess_wires.set(wires.len());
         // The overall "sheet" viewport now IS the paper view itself, so its own
         // border rectangle must not be drawn as an entity on the sheet.
         let sheet = self.current_layout_sheet_viewport_handle();