feat(isb1): add KV cache stress benchmark with multi-turn synthetic traces

Add ISB-1 (Inference Stress Benchmark) — a multi-turn, long-context KV cache
stress testing dataset for InferenceX V3.

## What this adds

**35 synthetic multi-turn traces** across 7 context bands (8K → 1M+ tokens):
- 6 workload families: long_chat, coding, agent, rag, cache_stress, multimodal
- KV stress patterns: prefix reuse, offload cliff, compaction, reactivation, fanout
- Real conversation content with 60-95% prefix overlap (enables prefix cache testing)
- Context assets from 15KB to 6.6MB inlined into traces for honest token counts

**Export bundles** for vLLM + SGLang replay:
- extension_131k: DeepSeek-R1, GPT-OSS, Qwen 3.5 (H200/B200)
- preview/long_context_500k: Qwen 3.5 500K context stress test
- preview/long_context_1m: Qwen 3.5 1M context stress test

**10 KV stress sweep configs** (isb1-kv-stress-pr993.yaml):
- 3 models × 2 GPUs × 2 engines
- Sweep: 2→256 concurrent users × on/off/noprefix offload modes × 1800s

## Coexistence with kv-cache-tester

This dataset complements PR #993's kv-cache-tester (522 real Claude Code traces):
- kv-cache-tester: real workload distribution, natural performance profile
- ISB1: controlled KV stress patterns that force offload cliffs and cache pressure

No files in experimental/multiturn/ are modified. Separate config files, separate
data directory (datasets/isb1/), shared replay infrastructure.

## Benchmark infrastructure
- benchmark_export_replay.py: replay harness with actual_context_len telemetry
- process_result_isb1.py: result aggregation with KV metrics
- Prometheus metrics: kv_cache_usage, prefix_cache_hits, kv_offload_bytes
- Pareto frontier: throughput vs p99 TTFT at each concurrency level

## Why this matters (from GTC 2026)
> "Right now the benchmarks are kind of showing the worst the chips will
> actually perform... for V3 we want to add agentic benchmarks like really
> good representative multi-turn QA chat benchmarks where there are a ton
> of client sessions each with multiple turns and we'll enable prefix caching."
> — Cameron Quilici

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: OCWC22

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+datasets/isb1/exports/preview/long_context_1m/*.json filter=lfs diff=lfs merge=lfs -text
+datasets/isb1/exports/**/*.json filter=lfs diff=lfs merge=lfs -text