Add streaming provenance release gates

[WaffleBits]

Summary

• add streaming token-event ingestion for mirrored backend observations
• report candidate route/scheduler provenance and streaming trace coverage in schema v4 release reports
• add a checked vLLM-to-SGLang streaming fixture plus regenerated gate artifacts

Validation

• cargo fmt --all --check
• cargo test --all-targets
• cargo clippy --all-targets --all-features -- -D warnings
• git diff --check
• cargo run --release -- mirror-gate --input fixtures/backend_mirror_streaming_vllm_sglang.json --output artifacts/backend-mirror-streaming-report.json