fix(editor): collapse the workspace split when the last buffer closes (T-459)

EditorRegistry.close() guarded its active-changed emit on `_activeId !=
null`, so closing the LAST buffer (active clears to null) emitted only
editor.closed — never the active-changed(id:null) the editor extension
listens for to call closeEditor(). editorOpen stayed true and the top
split sat orphaned over the Claude pane. Always emit active-changed when
the active buffer is removed, including the cleared-to-null case; the
slot renderer already collapses correctly once editorOpen flips false.

The existing extension test fabricated the null active-changed event, so
it passed despite the registry never emitting it — that gap is why the
bug shipped. Add a registry test that drives the real close() path, plus
a slot_host widget test asserting the split (drag handle) drops out and
the primary pane fills the column.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -25,6 +25,10 @@ heading, and (b) bumping `pubspec.yaml` `version:` in the same commit.
 - **Default window opens larger (1600×900) on Linux.** 720p was short enough
   that the welcome screen's version/theme footer overlapped the tips card;
   the taller default clears it, matching the macOS default. (T-477)
+- **Editor split collapses when the last buffer closes.** Closing the final
+  editor buffer left the top split orphaned over the Claude pane; the daemon
+  now emits the buffer-cleared event it was suppressing, so the split drops
+  out and the primary pane fills the column. (T-459)
 
 ## [2.8.0] — 2026-06-19