docs(redact/server): correct fallback wire-format comment — fallback '<div>' is visible, not hidden

[sukvvon]

Summary

• Comment vs. emit drift: bootstrap-script.ts's wire-format block comment claimed <!--$?ID--><div hidden id="B:ID">fallback</div><!--/$-->, but the actual emit at walk.ts:441 writes <div id="B:ID"> (no hidden). The adjacent comment at walk.ts:438 already states "Visible div wrapper so the fallback UI shows".
• Why visible is correct: the B: div wraps the user-visible loading state. Hiding it would defeat the purpose of streaming SSR fallbacks. Only the S: resolved-content staging div is hidden, so $RC(ID) can splice its children into place before the B: div is removed.
• Doc-only: comment text only; no behavior change.

Test plan

• grep <div .* id="B: repo-wide — only walk.ts:441 emits, and it has no hidden
• cross-checked against the runtime: $RC (bootstrap-script.ts:30-39) and hydrateSuspenseBoundary (suspense/full.ts:141-148) both assume a visible B: container
• pnpm test:types — clean