Files: a narrow listing keeps its names; a column starts fresh for a new folder

The directory listing's size and date columns are fixed, so at 280px the
name had no room and rows showed no names. The heading and rows now sit
in a sideways scroll box whose child keeps the least width: the name's
least, the fixed columns and each row's icon, gaps and padding. A column
of the explorer was keyed by its depth, so a new folder there inherited
the old list's cursor; columns and rows are keyed by their folder. The
explorer asserts each folder on its path is a folder in its parent.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

TASKS.md

@@ -323,3 +323,4 @@ Per-component lines live in `tasks/`. Tags there:
 | T26e | 2 | PASS | |
 | T27a | 1 | FAIL | a tile's long name leaned on gpui's line_clamp, which cut its second line at both ends and lost the extension |
 | T27a | 2 | PASS | |
+| T27b | 1 | FAIL | the listing's fixed columns left no room for names at 280px; a column keyed by its depth handed its cursor to the next folder there |