Canvas · Graphs and boards: a field hands focus back on Enter and Escape

writing::begin records the owner's focus, the mind map's or the tool
layer's, through forms::Editing::returning. The field keeps its words on
Enter's release and drops them on Escape, and both hand focus back, so
the map's arrows and the board's Backspace work at once after a write.

TASKS.md

@@ -379,3 +379,4 @@ Per-component lines live in `tasks/`. Tags there:
 | T32b | 2 | PASS | |
 | T32c | 1 | PASS | |
 | T32d | 1 | FAIL | Tab in a mind map's rename field added a topic; the replaced wire, the connector released on its own shape and a deleted shape's links went untested |
+| T32d | 2 | FAIL | Enter or Escape in a canvas field handed focus nowhere, so the map's and the layer's keys went dead until the next press |