Operator role attestation flow

**Problem Statement.** Operator is rotated via 2-step hand-off.
There is no off-chain provenance marker for who proposed the rotation
on-chain.

**Expected Outcome.**
- Add a `propose_operator_with_provenance(caller, new_op, memo)` storing a
  memo (limited size).
- Proposal state includes a free-form memo field.
- Tests cover memo transmission.

**Files:** `lib.rs`, tests.