fix(share): stop leaking content-derived turn ids in published snapshots#394
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Turn ids from ensureTurnIds are fnv1a32 content hashes; emitting them in turns[].id / turn_order / hidden_turns let any reader of the public snapshot JSON fingerprint content and enumerate which turns the author excluded. Remap ids to opaque positional tokens before upload. Also: external links in rendered markdown now open in a new tab with rel="noreferrer noopener", and decodeSnapshot warns (best-effort decode) on an unknown future schema_version instead of trusting it silently. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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What
Remap every turn id to an opaque positional token (`t0`, `t1`, …) before a snapshot is uploaded, so neither `turns[].id`, `turn_order`, nor `hidden_turns` carry content-derived values. Also: external links in rendered markdown now open in a new tab with `rel="noreferrer noopener"`, and the reader's `decodeSnapshot` best-effort decodes (and warns) on an unknown `schema_version` instead of trusting it silently.
Why
Turn ids came from `ensureTurnIds`, an fnv1a32 hash of the turn body. Emitting them on the public snapshot let any reader of `/api/snapshots/` fingerprint content (reversible for known content classes) and enumerate exactly which/how many turns the author excluded — contradicting the blank-body guarantee for hidden turns. The markdown links lacked `rel`/`target`, allowing top-frame navigation and a window.opener handle.
How it connects
The reader only uses ids for ordering and hidden-lookup, never for content, so opaque positional ids are fully compatible. Adds a regression test that fails on the pre-fix code (asserts no content-hash id appears in the emitted snapshot) plus link-hardening and unknown-version tests.