starknet: add dedicated admin signer rotation for bridge signature verification

[omarespejel]

Summary

This issue tracks Starknet signer-rotation hardening for OmniBridge signature verification.

Scope

• starknet/src/omni_bridge.cairo
• starknet/src/bridge_types.cairo
• starknet/tests/test_contract.cairo

Motivation

The bridge signer (omni_bridge_derived_address) is constructor-set and currently has no dedicated rotation path. Operationally, a direct admin-governed rotation function is needed for controlled key migration/incident response.

Proposed Changes

• Add admin-only setter for omni_bridge_derived_address.
• Emit signer-rotation event with old/new signer and admin actor.
• Add tests for:
  • authorized rotation
  • unauthorized rotation rejection
  • successful fin_transfer with signature from rotated signer

Acceptance Criteria

1. Admin can rotate signer address via dedicated method.
2. Non-admin rotation attempts are rejected by access control.
3. Post-rotation fin_transfer validates signatures from new signer.
4. Starknet test suite passes.

[omarespejel]

Tracking under epic #554 (Starknet OmniBridge Security Hardening Program).