Reject duplicate content in restored split layouts

[bb-slop-cop]

Human comments

What was wrong

The restored split-layout boundary accepted unique pane IDs even when multiple panes carried the same logical content. The split renderer then mounted a complete thread view for every accepted node, duplicating all thread-owned chrome. The direct reproduction and root-cause report is https://get-bb.github.io/reports/issues/3062.html.

What changed

The split-layout parser now rejects duplicate logical pane content using the same identity rules as live pane navigation. Invalid restored state falls back to the current route's single pane. This changes no wire protocol, public API, dependency, migration, or stored-data format.

How you verified

• Added a component regression that restores four unique pane IDs with the same thread content.
• Before the production change, the focused suite failed with expected length 1, received 4 while 48 tests passed.
• After the production change, the focused suite passed all 49 tests.
• The persistence and split-area suites passed all 49 tests.
• pnpm exec turbo run typecheck --filter=@bb/app passed.
• git diff --check passed.
• The diff contains 48 changed text lines across two files and no binary changes.
• The pre-push EAP model-name scan found no matches in the working tree, tracked files, added lines, or commit messages.

Fixes #3062

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