R8.1: document content keeps the file's colours in a dark app

Pages, cells, slides and PDF pages paint as the file says in any
appearance; only the chrome around them follows dark mode. The
kernel keeps Ely's one light scheme, sent once a view, and MuPDF
no longer inverts. Reviews now run gpt-6.1-sol at xhigh.

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

crates/ely-pdf-engine/tests/edit.rs

@@ -257,7 +257,7 @@ fn long_clashing_field_names_still_get_a_number() {
 
 /// Red pixels of a page at one pixel a point.
 fn red(worker: &mut Worker, doc: Doc, page: usize) -> usize {
-    let pixels = worker.render(doc, page, (1.0, 0), [0, 0, 612, 792], false).expect("renders").bgra;
+    let pixels = worker.render(doc, page, (1.0, 0), [0, 0, 612, 792]).expect("renders").bgra;
     pixels.chunks_exact(4).filter(|p| p[2] > 180 && p[1] < 80 && p[0] < 80).count()
 }