Implement barrier collective primitive

[Copilot]

Adds barrier() synchronization primitive to complete the core QMPI collective operations API.

Implementation

• Abstract interface: Added barrier() to CollectiveComm base class
• Centralized algorithm: Rank 0 coordinates by collecting arrival messages from all ranks, then broadcasting release
• Message validation: Headers validated to ensure protocol correctness ("BarrierArrival" / "BarrierRelease")
• API integration: Exposed via QMPICommunicator.barrier()

Usage

from netqmpi.sdk.communicator import QMPICommunicator

def main(app_config=None, rank=0, size=1):
    COMM_WORLD = QMPICommunicator(rank, size, app_config)
    
    with COMM_WORLD:
        # Phase 1: Each rank does independent work
        q = COMM_WORLD.create_qubit()
        q.H()
        
        # Synchronization point
        COMM_WORLD.barrier()
        
        # Phase 2: All ranks have completed phase 1
        q.measure()
        COMM_WORLD.flush()

Classical message passing only - no quantum operations required.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Implement additional collective primitives (barrier, etc.)</issue_title>
<issue_description>## Description

Files: netqmpi/sdk/primitives/collective/, netqmpi/sdk/communicator/communicator.py

Current primitive status

Primitive	Status
qsend / qrecv	✅
qscatter / qgather	✅
expose / unexpose	✅ (telegate)
qreduce	❌
barrier	❌

Implementation priority

2. barrier - Basic synchronization between nodes
3. qreduce - For quantum reduction operations

Impact

• Severity: Low (additional functionality)
• Improvement: QMPI API completeness</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Implement additional collective primitives (barrier, etc.) #9

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.