* fix: reject unsupported pool expansion with actionable errors
Report singleton-pool and persisted-topology constraints before misleading startup retries. Preserve single-node multi-drive admission and existing parity policies, and cover format preservation plus operator recovery guidance for issue #6186.
* fix: keep pool layout errors typed
Preserve actionable pool layout diagnostics without adding generic formatted errors. Tighten the shrink-only baseline and assert that both typed payloads survive the I/O boundary.
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Co-authored-by: houseme <housemecn@gmail.com>
cxymds