docs: update the verification contract once ledger anchoring ships

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Context

Sub-issue of #9267 (external decision-ledger anchoring). Once anchoring ships (#9269-#9274), the "tamper-evident, not tamper-proof" framing published in What you can verify (row 1, from #9186) and stated in migrations/0180_decision_ledger.sql's own header comment goes stale — the honest boundary this repo publishes must track what's actually shipped, not lag behind it.

Requirements

1. Update row 1 of /docs/what-you-can-verify: the ledger-integrity claim upgrades from "tamper-evident, self-operated" to "tamper-evident, externally anchored via Rekor + git-commit (cross-verified by GH Archive/Software Heritage)" — state precisely what this newly closes (wholesale re-chaining is now independently detectable) and what it still doesn't (see next point). Present the Bittensor on-chain anchor (ledger: Bittensor on-chain commitment backend — optional, Gittensor/SN74-audience corroboration #9277) as clearly-labeled optional, Gittensor/SN74-audience corroboration in its own callout, never folded into the default claim every verifier is told to check.
2. Be precise about what anchoring does and does not prove. Anchoring proves a specific (seq, rowHash) existed at a specific time in an external, independently-checkable record. It does not, on its own, prove every record between anchors wasn't tampered with and then re-anchored consistently — the existing verify endpoint's full-chain walk is still what closes that, anchoring bounds how far back a rewrite could reach undetected.
3. Publish the end-to-end verifier walkthrough from the orb(trust): external decision-ledger anchoring — Rekor + git-commit, from tamper-evident to tamper-proof #9267 research (fetch anchors, verify against the published key, bind to the live chain via ledger: GET /v1/public/decision-ledger/row/:seq — bind an anchor back to the live chain #9269's row endpoint, walk the chain) as runnable commands, mirroring verify-this-review.mdx's existing style.
4. Update migrations/0180_decision_ledger.sql's header comment and src/review/decision-record.ts's matching doc comments — both currently state the anchoring gap in the present tense; they need to describe what actually exists once this ships, not leave stale "not yet built" language sitting next to shipped code.

Deliverables

• Docs page updated with the new claim, its precise boundary, and the runnable verifier walkthrough
• Both in-code doc comments (migration + decision-record.ts) updated to match reality
• Docs drift/nav tests still green

Expected outcome

The published verification contract never claims less — or more — than what's actually running.

References

Sub-issue of #9267. Depends on #9269-#9274 (the anchoring work itself) all landing first. Docs precedent: apps/loopover-ui/content/docs/what-you-can-verify.mdx, verify-this-review.mdx.

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JSO-6438