T21a round 2 fixes

- a field notes its selection when a key goes down, so undo of typing selects
  what the typing replaced
- reading order skips table cells merged away, so Up and Down across blocks
  land on cells that show

TASKS.md

@@ -248,4 +248,5 @@ Per-component lines live in `tasks/`. Tags there:
 | T20 | 1 | FAIL | the demo picked past memory's end; an empty gutter had no height to press; a breakpoint's box also opened it; zoomed ticks lost their places; docs promised bound keys and batch actions |
 | T20 | 2 | PASS | |
 | T21a | 1 | FAIL | Enter kept a selection; undo of a split left focus on a gone field; italic inside bold ate the bold; keys went into merged cells; undo kept a changed kind's highlighter |
-| T21a | 2 | — | pending review |
+| T21a | 2 | FAIL | undo of typing lost the selection it replaced; Up from below a table landed in a hidden merged cell |
+| T21a | 3 | — | pending review |