feat: Add P95 latency metrics alongside P99 system-wide

[akhilpathivada]

📋 Summary

Adds P95 (95th percentile) latency metrics alongside existing P99 metrics across all VectorDBBench components for more comprehensive performance analysis.

🎯 Motivation

• P95 provides a middle ground between P99 (slowest 1%) and average latency
• More actionable for optimization compared to P99 alone
• Industry standard metric for performance monitoring

🔧 Key Changes

• Backend: Added P95 calculation in serial, concurrent, and streaming runners
• Frontend: Added P95 selection option in streaming and concurrent pages
• Results: Added P95 column to results table and exports
• Backward Compatibility: Existing result files show 0.0 for P95 metrics

🎮 Usage

# Streaming/Concurrent pages now offer P95 option
latency_type = st.radio("Latency Type", options=["latency_p99", "latency_p95", "latency_avg"])

Results Table:

latency(p99)	latency(p95)	recall
12.5ms	8.3ms	0.95

🧪 Testing

• All files compile successfully
• Backward compatibility with existing result files
• No breaking changes to existing APIs

📊 Impact

Generic support across all database clients and test cases. Users can now choose between P99 and P95 latency metrics based on their analysis needs.

[akhilpathivada]

/assign @XuanYang-cn @alwayslove2013

[sre-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: akhilpathivada, alwayslove2013
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