fix ClidePane notifying focus listeners during build

ClidePane.didChangeDependencies/didUpdateWidget run in the build phase and
called FocusTracker.setStatusWidget -> notifyListeners() synchronously,
rebuilding the focus-listening status-bar item mid-build — Flutter threw
"markNeedsBuild called during build" on every frame once a Claude pane was
focused. The convey now defers to a post-frame callback when mid-build
(re-checking focus then) and applies immediately otherwise. The T-150
widget tests missed this because no focus listener was in their tree;
added a regression test with PaneContextStatusItem present.

T-159.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -145,6 +145,10 @@ heading, and (b) bumping `pubspec.yaml` `version:` in the same commit.
 
 ### Fixed
 
+- Claude pane no longer floods the console with "markNeedsBuild called
+  during build" (T-159) — a focused pane surfacing its status-bar widget
+  now defers the notification out of the build phase instead of rebuilding
+  the status item mid-build.
 - The git / tickets / decisions / pql / problems tabs no longer log
   `i18n: namespace not registered` on boot (T-155). An extension's
   localized tab title is now loaded automatically on activation, and the