Files · Items: one icon map, file icon, file row and file grid

The crate had two maps from a file's name to its icon, in chat and in the
file tree, and they disagreed. One map now lives in primitives, over the
Lucide file-* icons, and chat, agent, the file tree and the directory
listing use it. FileIcon draws it for a file or a folder. FileItem is a
file's row, a ListItem with its size and when it changed. FileList is the
directory listing. FileGrid lays tiles in the order given, with a cursor
seeded from the owner's pick: arrows by tile and by row, Home, End, Enter
and a double press to open. The count of what fits a width has one home,
layout's columns_for, and fit builds on it.

src/primitives/mod.rs

@@ -1,6 +1,7 @@
 mod backdrop;
 mod disclosure;
 mod divider;
+mod files;
 mod focus;
 mod ghost;
 mod icon;
@@ -13,6 +14,7 @@ mod tooltip;
 pub use backdrop::{Backdrop, Place};
 pub use disclosure::Disclosure;
 pub use divider::Divider;
+pub(crate) use files::file_icon;
 pub use focus::{FocusNext, FocusPrev, FocusRing, FocusScope};
 pub(crate) use focus::{Takeover, give_back, hand_back, hold_focus, tab_stop, take_focus};
 pub use ghost::DragGhost;