T26a Media: images

A new chapter, src/media. ImageViewer fits a picture to its stage and
zooms by the document steps, by keys, buttons and Cmd-scroll about the
pointer; a drag through gpui's drag, a scroll or the arrows pan it once it
overhangs; R turns it a quarter at a time. gpui paints pictures without a
transform, so media::turn turns the decoded frames off the main thread,
an asset keyed by the picture and the turn; `image` 0.25, which gpui
already decodes with, is now a dependency. ImageThumbnail shows a picture
in its own shape with a corner label and a ring when chosen, and is a Tab
stop only with a handler. Lightbox / Gallery Viewer points to
overlays::Lightbox, which the gallery opens from the thumbnails.

T26 splits into T26a–e. AGENTS.md: the drag rule says drags that pan,
turn or slide go through gpui's drag; pictures turn by their pixels.

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

examples/gallery/pages/mod.rs

@@ -14,6 +14,7 @@ mod generative;
 mod git;
 mod layout;
 mod lists;
+mod media;
 mod menus;
 mod motion;
 mod navigation;
@@ -65,6 +66,7 @@ pub const ALL: &[Page] = &[
     chat::PAGE,
     agent::PAGE,
     generative::PAGE,
+    media::PAGE,
     theme::PAGE,
 ];