feat(server): vmafx-server in Go (gRPC + HTTP, observability)

[lusoris]

Summary

• Adds cmd/vmafx-server — a production Go binary serving VMAF scoring over gRPC (VmafxScoring service, proto/vmafx.proto) and HTTP/JSON (/healthz, /readyz, /metrics, POST /v1/score) with full endpoint parity to the Python server from PR chore(deps): Update dependency openai to >=3.19.2 #1583.
• pkg/libvmaf/ — cgo wrapper (New/Score/Close) backed by the vmaf CLI binary; local-dev links core/build-cpu/src/libvmaf.so, production uses LD_LIBRARY_PATH.
• pkg/observability/ — log/slog JSON logger, Prometheus registry (vmafx_server_* counters + latency histogram), 30 s SIGTERM graceful shutdown.
• Dockerfile.go-server — multi-stage: golang:1.23-bookworm builder → gcr.io/distroless/cc-debian12 runtime; EXPOSE 8080 + 50051.
• Helm values.yaml image updated to ghcr.io/vmafx/vmafx-server.
• Python compat layer from PR chore(deps): Update dependency openai to >=3.19.2 #1583 retained; Stage-3 cleanup PR to follow.
• ADR-0703 (child of ADR-0701).

Deliverables checklist

• Research digest: no digest needed — this is a greenfield implementation with a clear language/protocol decision in the task specification.
• Decision matrix: docs/adr/0703-vmafx-server-go-grpc.md § Alternatives considered (5 alternatives with pros/cons/why-not).
• AGENTS.md invariant note: no rebase-sensitive invariants — cmd/, pkg/, gen/, proto/, go.mod are fork-only paths with no overlap with upstream Netflix/vmaf.
• Reproducer / smoke-test command (see below).
• changelog.d/added/vmafx-server-go.md
• docs/rebase-notes.md — fork-only; no upstream impact.

Smoke-test commands

# Unit tests (no vmaf binary required — shell stub)
go test ./cmd/vmafx-server/... ./pkg/libvmaf/... -v

# Build + run (requires core/build-cpu to exist)
go build ./cmd/vmafx-server
./vmafx-server \
    --vmaf-binary core/build-cpu/tools/vmaf \
    --model-dir   model/ \
    --port 8080 --grpc-port 50051 &

# Health check
curl -s http://localhost:8080/healthz
# → {"status":"ok"}

# Readiness check
curl -s http://localhost:8080/readyz
# → {"status":"ready"}

# Metrics (Prometheus exposition)
curl -s http://localhost:8080/metrics | grep vmafx_server

# Score (Netflix golden pair — expect ≈ 76.6683)
curl -s -X POST http://localhost:8080/v1/score \
  -H "Content-Type: application/json" \
  -d '{"reference":"python/test/resource/yuv/src01_hrc00_576x324.yuv","distorted":"python/test/resource/yuv/src01_hrc01_576x324.yuv","model":"vmaf_v0.6.1"}'

# gRPC (requires grpcurl)
grpcurl -plaintext \
  -d '{"reference":"python/test/resource/yuv/src01_hrc00_576x324.yuv","distorted":"python/test/resource/yuv/src01_hrc01_576x324.yuv"}' \
  localhost:50051 vmafx.v1.VmafxScoring/Score

State.md / bug tracking

No bugs opened or closed. Row added to docs/state.md Recently closed:
T-VMAFX-SERVER-GO-GRPC-2026-05-28.

FFmpeg-patches impact

No changes to core/include/libvmaf/, public C-API headers, or CLI flags.
ffmpeg-patches series not affected.

🤖 Generated with Claude Code