TASK 64 units B and C — the two pure halves, no wiring yet.
- `Services/ChromaKeyDetector.cs`: samples the frame's outer ring (the screen is at the
edges, the subject in the middle), reports Green/Blue/None plus the colour actually shot
and a confidence. Confidence is coverage × hue tightness, which is what keeps a lawn or
a football pitch from being offered as a keyable screen. Letterbox bars are excluded from
the denominator rather than counted against the screen. Never decides anything: an offer
is the creator's to accept.
- `Services/VideoFilterChain.cs`: options → `-vf` (scale → colorkey → despill → format=bgra)
plus a `Signature` so two layers keyed identically can share one decoder. Numbers clamp and
an unparseable colour falls back to the app default, so ffmpeg is never handed a filter it
rejects at spawn.
- Keying is done by the bundled ffmpeg, not by us: verified against the shipped binary that
`colorkey=…,format=bgra` emits straight-alpha BGRA (alpha 0 on keyed pixels), which is what
the compositor already blends. No per-pixel loop lands in the 16.67 ms render budget.
Test: `ChromaKeyDetectorTests` — green, blue, letterboxed, lawn (must say None), mixed scene,
dark frame, plus the chain strings and signature sharing.
Docs: TASK 64 queued in TASKS.md with its slice table; the decisions.md chroma-key reversal
recorded as a reversal (it was buried for webcam background removal, not for supplied clips).
