docs: add multisig authority rotation guide for Stellar

[Mathew2k-hash]

Multisig authority rotation guide for Stellar

Adds an operational runbook for custodians rotating signer keys on Stellar multisig accounts —
picking up where the existing Recovery Scenarios section in stellar-multisig-withdrawal.mdx
leaves off.

What's new

• Zero-downtime staged rotation — add the replacement signer before removing the retiring one,
  keeping the account continuously authorized. Includes addSigner, verifySignerWeight, and
  removeSigner helpers with the overlap-window tradeoffs explained.
• Emergency rotation — two paths: fast single-tx revocation when remaining good signers cover
  the high threshold, and an atomic break-glass add+remove for when they don't. Includes a
  follow-up note on moving funds after any compromise.
• Auditor trail — block-by-block table showing what a rotation looks like in the ledger and
  Wraith announcement stream, a getSignerSetAtLedger Horizon query pattern, and
  getAnnouncementsAroundRotation for correlating withdrawals to a signer epoch.
• End-to-end futurenet script — full 2-of-3 → fresh 2-of-3 rotation: account setup, three
  staged add/verify/remove cycles, final signer-set assertion, and annotated expected output.
• Operational checklist — pre-rotation steps (HSM storage, anti-clipboard-hijack review,
  testnet dry-run, monitoring setup).

Changes

┌──────┬────────┐
│ File │ Change │
├────────────────────────────────────────────────┼───────────────────────┤
│ guides/stellar/multisig-authority-rot │ New guide (709 lines) │
│ ation.mdx │ │
├───────────────────────────────────────┼──────────────────────────────────────────────────┤
│ guides/stellar-multisig-withdrawal.md │ cross-link added at top of Recovery │
│ .mdx │ Scenarios section │
├────────────────────────────────────┼─────────────────────────────────────────────────────┤
│ docs.json │ Nav entry added to Operations group after │
│ │ stellar-multisig-withdrawal │
└────────────────────────────────────┴─────────────────────────────────────────────────────┘

Tested

pnpm run check:snippets — 586 snippets checked, 0 failures.
closes #129

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[truthixify]

Merged. The staged add-then-remove rotation is exactly the gap the existing recovery scenarios left open. Nice work @Mathew2k-hash.