Theme · Platform: caption buttons of their own height, titles that give way

Windows caption buttons take the bar's height less its hairline, a new
token, instead of their row's, so they keep their height beside caps
that wrap below them. A title bar's title is an Ellipsis: on Windows it
takes the free width before the buttons and gives way; a centered title
sits inset past the lights on both sides. The controls never shrink.
Tests hold a long Windows title's close button inside a 280px bar and
wrapped caps inside their row.

TASKS.md

@@ -424,3 +424,4 @@ Per-component lines live in `tasks/`. Tags there:
 | T39a | 3 | PASS | |
 | T39b | 1 | FAIL | a long theme name pushed its badge past 280px; four-digit colors refused the theme |
 | T39b | 2 | PASS | |
+| T39c | 1 | FAIL | Windows caption buttons took their row's height; a long Windows title pushed the buttons past the bar |