Add 12 Phase 5 tests for the AECP/AEM entity model:
- READ_DESCRIPTOR for existing and non-existent descriptors
- AECP packet filtering (wrong EtherType, wrong subtype)
- Unsupported AECP message types (ADDRESS_ACCESS, etc.)
- Unimplemented AEM commands (REBOOT, etc.)
- ACQUIRE_ENTITY and LOCK_ENTITY for legacy mode
- Milan ENTITY_AVAILABLE, LOCK_ENTITY (lock/contention/unlock)
- Milan LOCK_ENTITY for non-entity descriptors
- Milan ACQUIRE_ENTITY returns NOT_SUPPORTED
- Milan READ_DESCRIPTOR
Also adds Milan test server helper with properly sized entity
descriptor for lock state, and AECP/AEM packet builder utility.
Updates avb-bugs.md with 3 new bugs found (bugs #6-#8).
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>