Refine benchmark reports and fix FFmpeg stderr draining

[jarcherNV]

Fold derived benchmark summary metrics into their base metric rows so the HTML report shows the median-focused view developers expect. Constrain the scenario table layout and collapse long command strings behind a details control to avoid wide commands crowding adjacent columns.

Drain FFmpeg stderr concurrently while streaming video frames to stdin to avoid subprocess pipe-buffer deadlocks during MP4 writing.

[greptile-apps]

Greptile Summary

Refines benchmark report presentation and prevents FFmpeg stderr pipe deadlocks.

• Adds parser provenance to benchmark metric metadata and uses it to fold only genuine perf-summary metrics.
• Preserves base metric medians when derived summary metrics share their base name.
• Simplifies metric tables to a median-focused view and constrains scenario table command rendering.
• Drains FFmpeg stderr concurrently while video frames stream to stdin.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains.

Important Files Changed

Filename	Overview
flashdreams/flashdreams/infra/runner_io.py	Concurrently drains FFmpeg stderr during frame streaming and joins the drain thread before processing diagnostics.
flashdreams/tools/benchmarks/harness.py	Persists per-metric record type, parser, and source provenance in scenario manifests.
flashdreams/tools/benchmarks/metrics.py	Labels perf-summary and profile-e2e records with explicit parser provenance.
flashdreams/tools/benchmarks/report.py	Uses provenance-gated metric folding, retains existing base statistics, and presents a constrained median-focused report layout.
flashdreams/tests/test_benchmark_harness.py	Covers parser provenance, safe metric folding, base-median preservation, suffix-like custom metrics, and report layout.
flashdreams/tests/test_runner_io.py	Verifies that stderr draining begins before FFmpeg frame writes proceed.

Sequence Diagram

sequenceDiagram
  participant Runner
  participant FFmpeg
  participant DrainThread
  Runner->>FFmpeg: Stream video frames through stdin
  FFmpeg-->>DrainThread: Emit stderr diagnostics
  DrainThread->>DrainThread: Accumulate stderr chunks
  Runner->>FFmpeg: Close stdin and wait
  Runner->>DrainThread: Join
  DrainThread-->>Runner: Complete stderr output

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