Implement appeal_link in the Soroban registry contract

[Ryjen1]

Problem

PaybeamEscrow::appeal_link is currently stubbed to return Error::NotImplemented (see contracts/stellink-escrow/src/lib.rs). The on-chain registry has no way to mark a link as appealed, even though the frontend already exposes the appeal action.

Proposal

Implement appeal_link so that either the link's creator OR a recipient address committed to the link record can mark a registered link as Appealed. Subsequent get_link calls return the appealed flag so off-chain UIs can suspend further state transitions.

Acceptance criteria

• Add an appealed: bool field to LinkRecord. Default false on register_link.
• Add an optional recipient: Option<Address> field to LinkRecord (set at registration time, used as one of the authorised appellants).
• register_link accepts the recipient address as a new parameter.
• appeal_link(env, link_id, caller):
  • Requires caller.require_auth().
  • Returns Error::LinkNotFound if the link doesn't exist.
  • Returns Error::Unauthorised unless caller == record.creator || Some(caller) == record.recipient.
  • Sets record.appealed = true and writes back to storage.
  • Emits an APPEAL event with (caller, link_id).
• Calling appeal_link on an already-appealed link is a no-op (idempotent), not an error.
• Add cargo tests covering: happy path (creator), happy path (recipient), unauthorised caller, unknown link, idempotency.

Files involved

• contracts/stellink-escrow/src/lib.rs
• contracts/stellink-escrow/src/test.rs

Notes

resolve_appeal is a separate issue (#6) — this issue only flips the flag. Resolution is the arbiter's job.