VLAN-subnet API (create/delete + CIDR alloc)

[CodeBuildder]

Context

Synthetic VLAN/subnet simulation with CIDR block allocation. Parent: #18.

Acceptance Criteria

• POST /vlans allocates a unique CIDR from the configured supernet
• CIDR overlap is detected and rejected with 409
• DELETE /vlans/{id} releases the CIDR back to the pool
• Concurrent CIDR allocation requests never produce overlapping blocks (concurrency test)

Dependencies

Blocked by: none | Blocks: none

Notes

No model tier. Concurrency-safe alloc.