Capture · Roots: roots drop w_full; hosts give the width

145 component roots drop w_full, the early returns included. Three
field rows keep it, Input, InputGroup and TagInput: stretched without
a width, taffy 0.9 laid the field's flex_1 share at no width in the
template picker at 280px, and the placeholder drew nothing. Container
keeps it, since mx_auto needs a width to fill.

Hosts that relied on a root's width now give it: MessageList boxes
each row at the list's width, the splash gives its bar the column's,
and test stages that mount a component as the window's root sit in a
size_full box. Tests hold a narrow row across the list and the splash
bar across its column; each goes red without its fix.

The gallery gains --narrow <px>, which lays each page in a padded
card at that width and shoots it without its script. The page
header wraps: the title takes the room and the theme switcher drops
below, where a long summary used to push it past the edge. A file
panel step waits for the panel to become key instead of a fixed
pause, which lost a race in a capture.

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

examples/gallery/pages/forms/mod.rs

@@ -114,7 +114,6 @@ const SCRIPT: &[Step] = &[
     Step::Shot("hotkey"),
     Step::DownAt("path", 380.0, 16.0),
     Step::UpAt("path", 380.0, 16.0),
-    Step::Wait(1200),
     Step::CancelPanel,
     Step::Wait(600),
     Step::DownAt("checks", 40.0, 66.0),
@@ -242,7 +241,6 @@ const SCRIPT: &[Step] = &[
     Step::Shot("gradient"),
     Step::DownAt("file-one", 100.0, 14.0),
     Step::UpAt("file-one", 100.0, 14.0),
-    Step::Wait(1200),
     Step::CancelPanel,
     Step::Wait(600),
     Step::Shot("files"),