Compose: list every case that composes a story whole in gap 20

docs/gaps.md now lists the same cases as docs/architecture.md and
reuse::plain. A doc comment on rejoin says that a run style an edit uses
for the first time stops the reuse until the old composition reaches the
same style at the same index.

crates/compose/src/reuse.rs

@@ -309,6 +309,11 @@ impl Reuse<'_> {
     /// At paragraph `pi` of the edited story, in the state `snap` with `styles` and `info` as
     /// they are: when the composition before the edit was in that state before the same
     /// paragraph, copy what it set from there on into `out`, `styles` and `info`, and say so.
+    ///
+    /// The copied lines keep their run-style indexes, so this needs `styles` to equal the old
+    /// table up to that point. A run style that the edit uses for the first time takes a new
+    /// index, and the story then composes on until the old composition added the same style at
+    /// the same index, or to its end. The result is the same; only the time differs.
     pub(crate) fn rejoin(
         &self,
         plan: &Plan,

docs/gaps.md

@@ -233,7 +233,10 @@ Kind: **F** feature · **U** UI/UX · **T** typography · **FF** file format ·
 ### 20. Performance on large documents (P)
 - **Missing:** the 8 ms keystroke budget in long stories: an edit that moves lines into other
   frames composes the story to its end, the lines an edit keeps are copied, and stories with
-  footnotes, tables, anchored objects or keep options that move lines compose whole; benchmarks
+  footnotes, endnotes, tables, anchored objects, named lists, tracked deletions, balanced
+  columns, vertical justification other than Top, column rules, type on a path, spanning or
+  split paragraphs, paragraph borders or shading, or keep options that move lines compose
+  whole; benchmarks
   against InDesign on long books and image-heavy catalogues.
 - **Have:** compose 300k characters ~38 ms warm, damage-region repaint (< 1 ms while typing),
   parallel story composition, and an edit composes only the paragraphs it can change in a plain