fix(security): fail closed on hostile clipboard throw values

[seonghobae]

Scope

Test-first repair for #349 on the deterministic local SafeClipboard adapter boundary only. No transport, persistence, auth, tenancy, credential, model/provider, network, deployment, retention, durable-audit, or clipboard-exfiltration authority is added.

Current state: hosted RED pending

• Base: protected main@e8109ec2a17de8bd6594487aa12c8c8a93cb2c03.
• RED commit: 758c36c05f104be0cc6b608acd5c753ea5b9b709.
• The new regression drives the real ProseMirror transformPastedHTML adapter with a host config getter that throws a Proxy whose getPrototypeOf trap throws a private sentinel.
• Protected production currently classifies the unknown thrown value with instanceof, so the expected hosted failure is the private sentinel escaping instead of '' plus a stable redacted invalid_html error.

Production code is intentionally unchanged until exact-head hosted RED is observed. After RED, the smallest repair will replace prototype inspection with a side-effect-free module-owned brand/check for genuine ClipboardSanitizationError values and normalize every other thrown value without inspecting it.

Integration boundary

Keep Draft/unmerged while #118 owns the unchanged exact protected v0.6.0 publication source identity.

Refs #349

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