feat(tune): vmafx-tune in Go (Stage 1 — compare subcommand)

[lusoris]

Summary

• Adds cmd/vmafx-tune/ — the Stage-1 Go port of vmaf-tune, installed as vmafx-tune-go alongside the Python binary during the migration
• Ports the compare subcommand: rate-quality sweep via VMAF-target CRF bisect for libx264 and libx265
• All other subcommands are stubs that redirect to vmaf-tune <subcommand>; Python tools/vmaf-tune/ is unchanged

New packages

Package	Description
pkg/encoder/	Encoder interface + LibX264Encoder / LibX265Encoder (subprocess ffmpeg)
pkg/bisect/	Stateless VMAF-target CRF bisect, score function injectable for unit tests
pkg/report/	EmitJSON (RFC 8259 strict, NaN→null) + EmitMarkdown renderers

JSON output is schema-v1/v2 compatible with the Python compare.py / report.py output.

Test plan

• go build ./cmd/vmafx-tune — clean build
• go test ./... -count=1 — all pass (bisect convergence, encoder mock, stub redirect, version flag)
• go vet ./... — clean
• vmafx-tune-go compare --reference src.mp4 --codecs libx264 --targets 60 --format json — produces valid RFC 8259 JSON with expected schema shape
• vmafx-tune-go ladder — exits non-zero, prints redirect to vmaf-tune ladder

Smoke-test reproducer

# Build
go build -o /tmp/vmafx-tune-go ./cmd/vmafx-tune

# Generate synthetic source
ffmpeg -f lavfi -i testsrc=size=320x240:rate=24:duration=2 -pix_fmt yuv420p /tmp/src.y4m

# Run compare (vmaf binary required for scoring; without it the row is ok=false)
/tmp/vmafx-tune-go compare \
  --reference /tmp/src.y4m \
  --codecs libx264 \
  --targets 60 \
  --format json

# Verify stubs redirect
/tmp/vmafx-tune-go ladder; echo "exit $?"

Deliverables checklist

• Research digest: docs/research/vmafx-tune-go-stage1-2026-05-28.md
• Decision matrix: ADR-0705 ## Alternatives considered
• AGENTS.md invariant note: cmd/vmafx-tune/AGENTS.md
• Reproducer / smoke-test command: see Smoke-test reproducer above
• changelog.d/added/vmafx-tune-go-stage1.md fragment
• docs/rebase-notes.md entry: no rebase impact (fork-only new directories)

State / ADR

• ADR-0705 accepted: docs/adr/0705-vmafx-tune-go-stage1.md
• docs/state.md: T-VMAFX-TUNE-GO-STAGE1-2026-05-28 row added to Recently closed

Out of scope (Stage 2+)

Hardware encoders (NVENC, QSV, AMF), tune-per-shot, ladder, fast, corpus,
report, HTML rendering, --workers concurrency cap, --score-backend GPU
auto-detection.

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ffmpeg-patches impact

no ffmpeg-patches impact: this PR adds a new Go binary under cmd/; it does not
touch any C API surface, public headers, meson_options.txt, or the ffmpeg-patches/
stack.

🤖 Generated with Claude Code