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

changelog.d/changed/cpp23-wave4.md

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+# C++23 Wave 4 — output writers (ADR-0733)
+
+Converted `core/src/output.c` → `output.cpp` as the Wave 4 instalment of the
+ADR-0708 C++23 internals migration. All four format writers (XML, JSON, CSV,
+MicroDVD subtitle) are now compiled under `-std=c++23` via an isolated
+`static_library` in `core/src/meson.build`.
+
+C++23 idioms applied:
+
+- **`LocaleGuard` RAII**: `vmaf_thread_locale_pop` is now called on destruction,
+  guaranteeing cleanup on all exit paths without requiring explicit `goto cleanup`
+  patterns.
+- **`std::string_view`**: score-format string parameter threaded through all
+  internal helpers, eliminating repeated implicit `strlen` calls.
+- **`[[nodiscard]]`**: all four public entry points (`vmaf_write_output_xml`,
+  `vmaf_write_output_json`, `vmaf_write_output_csv`, `vmaf_write_output_sub`)
+  now produce a compile-time diagnostic if callers discard the return value.
+- **`constexpr`**: `pool_method_name` and `DEFAULT_SCORE_FORMAT` promoted to
+  `constexpr`.
+- **`static_cast<VmafPoolingMethod>`**: explicit enum casts replace implicit
+  `unsigned → enum` conversions that were valid in C but ill-formed in C++.
+- **`nullptr`**: replaces `NULL` in all guard expressions.
+
+Public C ABI and output bytes are preserved exactly (verified via `test_output`
+and CLI smoke test against the Netflix golden YUV pair).
+
+Also removes stale `test_ansnr_simd` meson.build entries that blocked
+`meson setup` after the ansnr feature drop in ADR-0720.