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fix(signals): a boundary's fallback is instantiated once per show (#3992)

* fix(signals): a boundary's fallback is instantiated once per show (#3991)

The fallback was built inside the boundary's output pass, so every
re-derive of the output while it showed rebuilt it: a sibling verdict
reader re-running on the error re-ran the tree, which threw the same
error again and re-derived the output (two fallbacks for one failure);
a reset() whose retry was pending re-derived it through prune (a third);
the Retry button was replaced and focus lost. Build the fallback in a
computed of its own beside the output, with the output's id, and keep it
while the fallback shows; dispose it when content shows. A later error
reaches it through the error accessor, as intended all along (the
once-per-show pin in createErrorBoundary.test.ts held for a render
effect erroring directly, not for a memo between).

Size-Exception: CSR with Show/For/Loading/Errored 12.976 -> 12.99 KB,
observe-tier CSR 14.56 -> 14.59 KB, compiled CSR 25.465 -> 25.49 KB
(+146 B minified each); core, isPending/latest and render unchanged.
Accepted by the maintainer 2026-10-12.

Fixes #3991

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LVm3wseHt9wRTY8LfPiHmQ

* fix(signals): dispose the fallback once the content that replaces it is on screen

A held swap to content keeps the fallback on screen, live, until the
commit (S-14): the disposal is a queued run — this flush's effect phase
when the pass is the frame's, the landing when the frame parks — and a
pass back to the fallback before then finds it still built and keeps it.
Pinned on the S-14 shape; fails with an immediate dispose.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LVm3wseHt9wRTY8LfPiHmQ

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Co-authored-by: Claude <noreply@anthropic.com>

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