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>

AGENTS.md

@@ -107,7 +107,7 @@ Ely GPUI Component. A component library for GPUI, in light and dark.
 - Keyed state lives while its element renders in consecutive frames. An overlay rendered only while open starts fresh each time.
 - Long grids scroll in a `uniform_list`. gpui has no nearest scroll, so a key move up scrolls with `Top` and down with `Bottom` (`forms::glyphs`).
 - Masks reshape edits inside `TextInput::set_fit`, which sees the replaced range and the typed text. A diff after the fact cannot tell typed characters from kept ones.
-- Ids inside a reusable component carry its owner's id or `EntityId`, animation ids too. Twin ids share focus, click and animation state.
+- Ids inside a reusable component carry its owner's id or `EntityId`, animation ids too. Twin ids share focus, click and animation state. A child keyed by its place inherits the state of what stood there before; key it by what it shows (`files::FileExplorer` keys a column by its folder).
 - Text wraps at the width its box had when measured. A cross-axis `max_w` comes too late, and a flex column counts wrapped text as one line. Put prose in a plain block inside a flex row with `flex_1().max_w(..)` (`feedback::states`).
 - gpui 0.2.2 keeps a no-wrap line's first measure, taken at full width inside flex and scroll boxes, so `.truncate()` clips there instead of ending in an ellipsis. A line that may overflow is a `typography::Ellipsis`.
 - gpui 0.2.2's `line_clamp` wraps once and lays the rest on the last line, cut hard; centered, it loses both ends. A name whose ends matter is a `typography::MiddleEllipsis` on one line (`files::FileGrid`).