refactor(core): cpp23 Wave 4 — output writers (ADR-0733)

[lusoris]

Summary

• Convert core/src/output.c → output.cpp as Wave 4 of the ADR-0708 C++23 internals migration
• All four format writers (XML, JSON, CSV, MicroDVD subtitle) compiled under -std=c++23 via isolated static_library + override_options=['cpp_std=c++23']
• Remove stale test_ansnr_simd meson.build entries blocking meson setup after the ADR-0720 ansnr drop

C++23 idioms applied

• LocaleGuard RAII: vmaf_thread_locale_pop runs on destruction — cleanup guaranteed on all exit paths
• std::string_view: score-format parameter threaded through all helpers, eliminating implicit strlen per write call
• [[nodiscard]]: all four public entry points annotated
• constexpr: pool_method_name[] and DEFAULT_SCORE_FORMAT
• static_cast<VmafPoolingMethod>: explicit enum cast replacing implicit C unsigned→enum conversions
• extern "C" wrapper: feature/alias.h, feature_collector.h, thread_locale.h wrapped at include site (they lack their own guards)

Deep-dive deliverables

• Research digest: no digest needed: Wave 4 continues ADR-0708 playbook
• Decision matrix: ADR-0733 ## Alternatives considered (per-file split vs bundle; std::ofstream vs FILE*; std::format vs printf-style)
• core/AGENTS.md invariant: Wave 4 RAII pattern + extern "C" wrapper note + upstream rebase guidance
• Reproducer / smoke test (see below)
• changelog.d/changed/cpp23-wave4.md
• docs/rebase-notes.md entry

Per-surface docs

no user-discoverable surface change — internal refactor; output schema bit-equivalent (verified via smoke test below)

Smoke test

# Build (CPU only, no GPU deps)
meson setup build -Denable_cuda=false -Denable_sycl=false -Denable_vulkan=disabled
ninja -C build

# XML output smoke (5 frames, Netflix golden YUV pair)
build/tools/vmaf \
  -r python/test/resource/yuv/src01_hrc00_576x324.yuv \
  -d python/test/resource/yuv/src01_hrc01_576x324.yuv \
  -w 576 -h 324 -p 420 -b 8 \
  -m version=vmaf_v0.6.1 \
  --output /tmp/out.xml --xml --frame_cnt 5

# JSON output smoke
build/tools/vmaf \
  -r python/test/resource/yuv/src01_hrc00_576x324.yuv \
  -d python/test/resource/yuv/src01_hrc01_576x324.yuv \
  -w 576 -h 324 -p 420 -b 8 \
  -m version=vmaf_v0.6.1 \
  --output /tmp/out.json --json --frame_cnt 5

# Fast test suite (49/49 pass)
meson test -C build --suite=fast

docs/state.md

no bug opened/closed/ruled-out — pure internal refactor

Merge-train note

Waves 1–3 in flight (PRs #41, #43, #44, #45, #48, #51, #54). This PR is independent of all of them (touches only output.c/.cpp, output.h, meson.build, and test/meson.build). No Vulkan-drop (#47) overlap confirmed (gh pr diff 47 --name-only | grep output → no output).

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