Two hardenings on the chain outlet branch:
A throwing chain selector runs before its entry's SlotErrorBoundary
exists, so uncontained it blacked out the whole owner region and skipped
the remaining chain. It now degrades to a decline: reported via
console.error with the registrant identity, later entries still tried,
all-null/all-throw passes land on the owner fallback.
The elected entry's boundary is now keyed by entry identity: an unkeyed
boundary that failed on entry A survived a re-election and kept a
healthy entry B blacked out until the outlet unmounted. The key remounts
the boundary fresh whenever the election changes.
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