M7: performance: retained canvas renders, virtual layers panel, shared text layouts, cargo xtask bench (#35)

Large documents stay interactive: the canvas re-renders only what a pan uncovers or an edit
touches, the layers panel lays out only the rows in view, and per-frame work no longer walks the
whole page.

- Benchmark: `cargo xtask bench [SIZES]` builds synthetic pages (`figcraft_engine::synthetic`:
  screens with an auto layout header, a card group, button instances and shapes/text) at
  1k/10k/50k layers and prints engine/render timings (`figcraft-cli perf --layers 1000,10000`) and
  headless whole-UI frame timings (an ignored ui-egui test).
- Retained canvas rendering (`figcraft_render::Retained`, used by the render worker): views snap
  to whole device pixels, so a pan shifts the previous image in place and renders only the
  uncovered strips; an edit renders only where changed layers were and are
  (`Document::changed_ids` diffs the two versions skipping shared structure; ancestors that paint
  from their children's pixels - booleans, effects, masks - count whole; background blur reach
  and noise are allowed for). Zoom, page, options, font or page-level changes, or large changes,
  render the whole view as before. `ui.inspect` reports `canvas.renderWork`.
- The texture image is converted on the render thread, not the UI thread.
- Layers panel: only visible rows are laid out (stable ids while scrolling); rows are cached per
  document version, page, expanded set and filter.
- Frame labels and layout grids of off-screen top-level frames are skipped; the Prototype tab's
  connection sources are cached per document version.
- Text: the layout cache keys by a cheap fingerprint confirmed by `TextData` equality (no Debug
  formatting per lookup, 32k entries) and hands out shared layouts; a layout generation marks
  renders stale when fonts arrive. Overflowing text is culled by its glyphs, so it no longer
  vanishes while its box is off screen.

Before -> after (release, Windows, 1440x900 view, ms; 10k / 50k layers):
- move one layer, re-render @100%:     7.65 / 6.49  -> 0.18 / 0.81
- move one layer, re-render zoomed out: 11.95 / 24.10 -> 0.16 / 0.77
- pan 40 px @100%:                     7.53 / 6.82  -> 0.69 / 0.81
- pan 40 px zoomed out:                12.07 / 24.30 -> 1.62 / 4.17
- full render @100% / zoomed to fit:   7.78 / 44.22 -> 6.56 / 38.83 (10k @100%, 50k fit)
- UI frame, every screen expanded in layers: 6.16 / 39.07 -> 0.24 / 0.57
- UI frame, scrolling that panel:      6.11 / 40.18 -> 0.43 / 0.40
- UI frame, layers filtered by name:   1.03 / 6.92  -> 0.17 / 0.36
- UI frame, Prototype tab:             0.89 / 6.45  -> 0.18 / 0.80
- UI frame, idle:                      0.31 / 1.43  -> 0.11 / 0.39
Unchanged and within budget: single-layer edits through the engine ~0.01 ms, normalize after a
one-node edit < 0.01 ms, hit test 26 / 93 us. Left: save/load at 50k (~0.7 s each), zoomed-out
renders at 50k (~40 ms).

Tests: retained renders match full renders after pans, moves, recolours, hide/remove/add, undo,
masks, overflowing text, edits under a background blur and an inner-shadow/noise layer, a drop-
shadowed group, and a blur panned in from the view's edge; zoom/page/options/page background
render full; junk views don't panic. Layer-row and prototype-source caches follow edits, undo,
page switches, expansion and the filter. `changed_ids`, the text cache, and the perf CLI.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

xtask/src/main.rs

@@ -7,6 +7,7 @@
 //! - `version` print the version, or `version set X.Y.Z`
 //! - `icon`    regenerate the app icon PNGs/ICO from code (original artwork)
 //! - `wasm`    check the core crates and web app for wasm32
+//! - `bench`   release-build timings at 1k/10k/50k layers (`bench 1000,10000` for other sizes)
 
 mod icon;
 mod layers;
@@ -37,8 +38,9 @@ fn main() -> ExitCode {
         "version" => version(args.get(1).map(String::as_str), args.get(2).map(String::as_str)),
         "icon" => icon::generate(&root()),
         "wasm" => wasm(),
+        "bench" => bench(args.get(1).map_or("1000,10000,50000", String::as_str)),
         _ => {
-            eprintln!("tasks: ci, layers, assets, parity, version [set X.Y.Z], icon, wasm");
+            eprintln!("tasks: ci, layers, assets, parity, version [set X.Y.Z], icon, wasm, bench [SIZES]");
             task == "help"
         }
     };
@@ -58,6 +60,23 @@ fn wasm() -> bool {
     run(cargo().args(["check", "--target", "wasm32-unknown-unknown", "-p", "figcraft-engine", "-p", "figcraft-ui-egui", "-p", "figcraft-web"]))
 }
 
+/// Engine/render timings (`figcraft-cli perf`) and UI frame timings (an ignored test driving the
+/// whole UI headless), release builds, for each comma-separated document size in layers.
+fn bench(sizes: &str) -> bool {
+    run(cargo().args(["run", "--release", "-p", "figcraft-cli", "--", "perf", "--layers", sizes]))
+        && run(cargo().env("FIGCRAFT_BENCH_LAYERS", sizes).args([
+            "test",
+            "--release",
+            "-p",
+            "figcraft-ui-egui",
+            "--lib",
+            "perf_ui_frames",
+            "--",
+            "--ignored",
+            "--nocapture",
+        ]))
+}
+
 /// Every asset file must be listed in ATTRIBUTION.md.
 fn assets() -> bool {
     let root = root();