feat(observability): add NVTX tracing and Prometheus metrics

[BruceLoveDecimal]

Observability: NVTX tracing + Prometheus metrics
Adds a two-layer observability stack for RL-Kernel.

NVTX tracing (micro) — rlk:: ranges around every C++ operator binding plus Python stage ranges (rlk::score, rlk::train_step, …), visible on an Nsight Systems timeline. Opt-in via RL_KERNEL_NVTX=1; off by default (cached branch, no cost).

Prometheus metrics (macro) — a per-process /metrics endpoint exposing token throughput, rollout QPS, stage latencies, backend/fallback state, KV-cache fragmentation, peak memory, and training loss, plus a ready-made Grafana dashboard. prometheus-client is an optional extra; without it every call is a silent no-op.

Notes

Instrumentation is exception-transparent and per-stage (never per-token). Measured overhead +0.9% on an RTX 5090.
Existing StageResult.metrics dicts are unchanged.
Verified on GPU: CUDA extension builds, full suite green (572 passed), nsys shows the rlk:: ranges, live /metrics served.
Closes #72

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