Description: Implement homepage preview extraction so %% more %% truncates only the homepage card while normal article rendering keeps the full post and hides the token. Follow §Security Design only insofar as preview rendering must not leak alternate content paths.
Decision budget:
Junior can decide: how to split blocks by blank lines, and how preview cards mark continuation affordances.
Escalate to TL/PTL: whether nested markdown blocks count as one block or multiple blocks for the 5-block fallback.
Acceptance criteria:
Task
Homepage preview stops at %% more %%.
Task
Normal post render does not expose the token.
Task
Documents without %% more %% preview the first 5 blocks.