M8: 50k-layer zoomed-out renders and faster save/load (#63)

Canvas previews zoomed out to 50% or less draw levels of detail
(crates/render/src/lod.rs): subtrees under 2 device px are splatted as
their averaged paint colours (box filter, no paths or effects), text
under 4 px is greeked into bars, small solid rectangles are splatted and
rectangular clip layers are composited without the blend pipeline.
Image paints and masks keep the exact path. Exports, thumbnails and
every other render stay exact (Options::lod is off by default); render
bands are assembled with a row copy instead of a pixmap draw.

.figcraft documents are serialized straight into the compressor and
parsed straight from the text (a header pass reads version and image
index without building a value tree); layer and text fields at their
default values are left out (63 -> 22 MB of JSON at 50k layers, older
files still load); JSON members use deflate level 3.

At 50k layers: zoomed-to-fit render ~58 -> ~8 ms, save ~1.0 s -> ~0.09 s,
load ~0.86 s -> ~0.16 s.

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

apps/figcraft-cli/src/perf.rs

@@ -50,17 +50,22 @@ fn find(doc: &Document, root: NodeId, k: usize, pred: impl Fn(&figcraft_doc::Nod
     doc.subtree(root).into_iter().filter(|i| doc.node(*i).is_some_and(&pred)).nth(k)
 }
 
-fn render(doc: &Document, page: NodeId, v: &ViewState) -> bool {
-    let (ox, oy) = v.origin();
-    let view = figcraft_render::View { origin: (ox, oy), scale: v.zoom, width: VIEW_W, height: VIEW_H };
-    figcraft_render::render_view(doc, page, &view, &Default::default()).is_some()
+/// What the canvas renders with: a preview, with levels of detail when zoomed out.
+fn canvas() -> figcraft_render::Options {
+    figcraft_render::Options { lod: true, ..Default::default() }
 }
 
-/// Render `v` panned right by `dx` page units through the retained renderer.
+fn render(doc: &Document, page: NodeId, v: &ViewState, opts: &figcraft_render::Options) -> bool {
+    let (ox, oy) = v.origin();
+    let view = figcraft_render::View { origin: (ox, oy), scale: v.zoom, width: VIEW_W, height: VIEW_H };
+    figcraft_render::render_view(doc, page, &view, opts).is_some()
+}
+
+/// Render `v` panned right by `dx` page units through the retained renderer, as the canvas does.
 fn retained(r: &mut figcraft_render::Retained, doc: &std::sync::Arc<Document>, page: NodeId, v: &ViewState, dx: f64) -> bool {
     let (ox, oy) = v.origin();
     let view = figcraft_render::View { origin: (ox + dx, oy), scale: v.zoom, width: VIEW_W, height: VIEW_H };
-    r.render(doc, page, &view, &Default::default()).is_some()
+    r.render(doc, page, &view, &canvas()).is_some()
 }
 
 struct Table {
@@ -99,15 +104,17 @@ fn measure(table: &mut Table, col: usize, layers: usize) -> CliResult<()> {
     table.put(col, "load .figcraft", ms(t));
     drop(back);
 
-    // Rendering a 1440×900 viewport: 100% on the first screen, and zoomed out to fit the page.
+    // Rendering a 1440×900 viewport as the canvas does: 100% on the first screen, and zoomed out to
+    // fit the page (also exactly, as an export of the same area would).
     let bounds = doc.page_content_bounds(page).ok_or_else(|| failed("empty page"))?;
     let mut at100 = ViewState { vw: f64::from(VIEW_W), vh: f64::from(VIEW_H), ..Default::default() };
     at100.cx = f64::from(VIEW_W) / 2.0;
     at100.cy = f64::from(VIEW_H) / 2.0;
-    table.put(col, "render 1440×900 @100%", mean(5, |_| _ = render(&doc, page, &at100)));
+    table.put(col, "render 1440×900 @100%", mean(5, |_| _ = render(&doc, page, &at100, &canvas())));
     let mut fit = at100.clone();
     fit.fit(bounds, 1.0);
-    table.put(col, "render 1440×900 zoomed to fit", mean(3, |_| _ = render(&doc, page, &fit)));
+    table.put(col, "render 1440×900 zoomed to fit", mean(5, |_| _ = render(&doc, page, &fit, &canvas())));
+    table.put(col, "render 1440×900 zoomed to fit, exact (no LOD)", mean(3, |_| _ = render(&doc, page, &fit, &Default::default())));
 
     // The canvas keeps its last image: a pan renders the uncovered strips, an edit the layers'
     // areas.
@@ -116,7 +123,7 @@ fn measure(table: &mut Table, col: usize, layers: usize) -> CliResult<()> {
     // Zoomed out, but with content right up to the edges (a fitted page has margins).
     let mut out = fit.clone();
     out.zoom *= 2.0;
-    table.put(col, "render 1440×900 zoomed out (2× fit)", mean(3, |_| _ = render(&doc, page, &out)));
+    table.put(col, "render 1440×900 zoomed out (2× fit)", mean(5, |_| _ = render(&doc, page, &out, &canvas())));
     for (name, v) in [("pan 40 px @100% (retained)", &at100), ("pan 40 px zoomed out (retained)", &out)] {
         let mut r = figcraft_render::Retained::default();
         let _ = retained(&mut r, &doc, page, v, 0.0);