Generative: the video strip scrolls and reveals; tags wrap in their row

The timeline's scroll box is the flex row itself, so its reach is its
shots; a focused shot brings itself whole into view through
layout::bring_into_view, which table headers now share; Extend sits
with the caption under the strip. A voice's tags cap at the row's
width and wrap inside it.

TASKS.md

@@ -297,3 +297,4 @@ Per-component lines live in `tasks/`. Tags there:
 | T25b | 2 | PASS | |
 | T25c | 1 | FAIL | a canvas with no handlers still took Tab, the pen and the ring; strokes from the host went unchecked |
 | T25c | 2 | PASS | |
+| T25d | 1 | FAIL | the timeline's strip never scrolled, its child stretched to the box; a focused shot past the edge stayed hidden; many voice tags ran past a 280px row |