perf: typing waits out the pass, and a block as tall as before skips it

An edit's pass waits for 300 ms of quiet instead of running between keys, so
while the typing goes on only the keystroke itself costs. When the quiet comes,
edits that only put text into one textblock and left the content's bottom where
the last pass had it skip the pass: the block is as tall as before, so nothing
below it moved. A break inside the block still runs it. Measured at the top of
a 124-page document: 41 ms a keystroke in Chromium, no pass for a letter typed
and taken back. The dom run holds both.

docs/architecture/pagination.md

@@ -114,10 +114,15 @@ back from the other.
 every transaction (`apply`) and each spacer widget carries a `key` naming what it draws, so
 a pass that lands the same spacers keeps their DOM. Unmapped, an edit above them left every
 widget one position off and the view rebuilt all of them — measured at the top of a
-124-page document: 1.5 s a keystroke, 144 ms with the mapping. An edit within
-`EDIT_IDLE_MS` of the last pass waits for a pause in the typing (the first after a pause
-runs at once); a forced recalc or a layout-only write never waits. The dom run
-(`tests/dom/run.mjs`) holds the budget.
+124-page document: 1.5 s a keystroke, 144 ms with the mapping. An edit's pass waits for
+`EDIT_IDLE_MS` of quiet, so while the typing goes on only the keystroke itself costs; a
+forced recalc or a layout-only write never waits. When the quiet comes, the pass is skipped
+if every edit since the last one only put text into or took text out of one textblock
+(`textOnlyBlock`: a paragraph or heading, inside lists at most — a cell, a note or a frame
+reaches further than its own height) and the content's bottom is where the last pass left
+it: the block is as tall as before, so nothing below it moved. A break inside that block
+still forces the pass, since its line has rewrapped. The dom run (`tests/dom/run.mjs`)
+holds the keystroke budget and checks that a letter typed and taken back runs no pass.
 
 **Layout constants** (must stay in sync between `pageBreaks.ts`, `Editor.svelte`, and `editor.css`):
 - `PAGE_HEIGHT = 1123px` (A4 portrait), `PAGE_GAP = 20px`, `CYCLE = PAGE_HEIGHT + PAGE_GAP = 1143px`.