T24b round 1 fix

- AgentProgress splits its bar only for two steps or more; a task of one
  step draws a plain bar
- test a_task_of_one_step_draws_before_and_after renders 0 of 1 and 1 of 1

TASKS.md

@@ -275,3 +275,4 @@ Per-component lines live in `tasks/`. Tags there:
 | T24a | 1 | FAIL | tall arguments pushed the approval dialog's title and buttons off screen; a long tool name in a narrow column pushed the time and chevron out of the card; the dark capture closed the card the light one opened |
 | T24a | 2 | FAIL | a dialog body's children shrank to the capped column instead of scrolling: a 2000px body drew at 860px and a scroll moved nothing |
 | T24a | 3 | PASS | |
+| T24b | 1 | FAIL | a task of one step crashed at render: its bar asked for one segment, and a segmented bar needs two |