fix(quickshell): stop the bar twitching when a collapse finishes

Two causes, both outside the panel animation itself.

bodyBottom and bodyEndPadding are animated reals, so their sum spends the tail
of every transition on a fraction. A layout rounds that up, then drops a pixel
the moment the animation lands on its exact value — a 1px hop after the motion
has visibly finished. implicitHeight and targetHeight now round.

The rest was the surface. Hyprland animates layer-surface resizes (animations
enabled, `layers` left at its default), and the deferred shrink put that resize
exactly where the panel motion ended. The bar is now sized once to the expanded
rail via the new BarPanel.expandedHeight and never resizes; only exclusiveZone
tracks the state, so the desktop still reflows once per toggle, at the start.
That retires barHeight, the content-height handler and the 340ms shrink timer.

A surface that stays tall would swallow clicks across the screen while the rail
is collapsed, so input is masked to the panel row.

Instrumenting BarPanel per frame ruled the panels themselves out first: slug
and summary hold the same absolute y through an entire collapse.

The bar and scrim also get their own layer namespaces. Nothing depends on them
yet; they are the handle for a layerrule that would exempt the rail from
compositor animations without catching the launchers, which share the default
"quickshell" namespace and do want their fade.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HEtXvseVb5gwAtKNhFx2PU

dotfiles/quickshell/hyprchrome/widgets/ChromeBackdrop.qml

@@ -47,6 +47,7 @@ PanelWindow {
 
     visible: scrim.opacity > 0
 
+    WlrLayershell.namespace: "hyprchrome-scrim"
     WlrLayershell.layer: WlrLayer.Top
     WlrLayershell.keyboardFocus: WlrKeyboardFocus.None
     exclusionMode: ExclusionMode.Ignore