R4.4: export and copy read as the visual mode reads

HTML export, plain text and rich copy read each paragraph, heading,
cell and HTML block through the component's MarkdownReader (pin
1251221). Emoji, alerts, the TOC and inline tags match visual mode.
Footnotes sit in place as [n]; link titles are not written.

Three review rounds failed; round 3's one finding (a scan per
footnote over open marks) is fixed with a growth regression, unreviewed.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

apps/ely-mark/src/export/tests.rs

@@ -25,7 +25,7 @@ fn export_asks_for_a_format_then_writes_it(cx: &mut TestAppContext) {
     cx.run_until_parked();
     let html = std::fs::read_to_string(&out).expect("written");
     assert!(
-        html.contains("<h1>Note</h1>") && html.contains("<svg"),
+        html.contains("<h1 id=\"h-0\">Note</h1>") && html.contains("<svg"),
         "{html}"
     );
 }