Add Feature Extractor

---
name: add-feature-extractor
description: Scaffold new feature extractor (e.g. novel metric) with C source+header, registry entry, doc stub, smoke test. Produces no SIMD or GPU path — those come via /add-simd-path and /add-gpu-backend.
---
<!-- markdownlint-disable MD060 -->

# /add-feature-extractor

## Invocation

```text
/add-feature-extractor <name> [--type=full-reference|no-reference]
```

## Files created

| Path                                        | Purpose                                    |
|---------------------------------------------|--------------------------------------------|
| `core/src/feature/<name>.c`              | Scalar reference implementation            |
| `core/src/feature/<name>.h`              | Prototype                                  |
| `core/test/test_<name>.c`                | Smoke test (1 frame, fixed expected value) |
| `docs/<name>.md`                            | Metric documentation (inputs, range, refs) |

## Files patched

- `core/src/feature/feature_extractor.c`: registry row.
- `core/src/feature/all.c`: `#include "<name>.h"` + array entry.
- `core/src/meson.build`: new source in feature set.
- `core/test/meson.build`: register test.

## Template fills

- Extractor struct: `extract` pointer, name, type (FR/NR), features exposed.
- No SIMD / GPU variants wired: feature exists first as scalar reference;
  SIMD/GPU paths tested against it later.

## Guardrails

- Names collide-checked against existing registry.
- Smoke test must pass (non-NaN, finite) against sample YUVs in
  `python/test/resource/yuv/`.