fix runtime exceptions surfaced when the editor opens

Now that the editor split actually opens (T-197), it exposed latent
issues, plus a coincidental Claude-session crash in the same log:

- T-203: the _EditorDragHandle's slider Semantics had value +
  onIncrease/onDecrease but no increased/decreasedValue, so Flutter
  asserted on every semantics flush — add them. And opening the split
  reparented the Claude pane (direct child → Column/Expanded), tearing
  down its SelectableRegion mid selection-update ('selectable not in
  this registrar' / 'inactive element'); a stable GlobalKey on the
  workspace primary makes Flutter move the element instead.
- T-202: rate_limit_event.resetsAt arrives as a unix-epoch number but
  was cast `as String?`, throwing in the stream-json line parser. Accept
  a num (epoch) or an ISO string.

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

CHANGELOG.md

@@ -54,6 +54,12 @@ heading, and (b) bumping `pubspec.yaml` `version:` in the same commit.
 
 ### Fixed
 
+- Opening the editor split no longer floods exceptions: the resize handle's
+  slider semantics now carry increased/decreased values, and the Claude pane
+  keeps a stable identity across the reparent so its text-selection region
+  isn't torn down mid-update. (T-203)
+- A `rate_limit_event` whose `resetsAt` is a numeric epoch no longer crashes the
+  Claude session — it was cast as a string. (T-202)
 - Filter/search inputs show their hint as visible placeholder text, and the
   search-glass icon is now optional — so the Search tab's Find fields (search,
   replace, include/exclude globs) are distinguishable instead of four identical