Why: D1-D5 were settled by the user (deps in, full-fidelity pptx, OG
card plus optional embed, dual placement); the proposal status must
reflect that before implementation starts.
Evidence: decision section rewritten with the settled choices and the
accepted trade-off on D2 (fall back to outline level with evidence only
if no Vite-compatible renderer builds).
Impact: documentation only.
Refs: task feature/file-preview-caps.
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