feat(export): pick the video encoder from the host's hardware, with a libx264 fallback

[EtienneLescot]

Scoping PR — no implementation yet. The branch carries a single empty commit so the PR can exist; drop it when the real work starts.

The problem

poc-d3d/src/pipeline.rs hard-codes the AMD encoder in three places — :250, :637 and :755 — and refuses to continue without it:

let enc_name = CString::new("h264_amf")?;
...
bail!("h264_amf introuvable");

AMF is AMD-only. On an NVIDIA or Intel host, or on any machine without a hardware H.264 encoder, this path cannot open an encoder at all — native export fails outright rather than degrading.

Development happens on an AMD GPU, which is exactly why this has stayed invisible.

What already exists

electron/media/ffmpegCapabilities.ts already probes the ffmpeg binary for its real encoder list, and its tests cover h264_qsv, h264_amf, h264_nvenc, h264_videotoolbox and libx264. The discovery is there — the Rust pipeline simply never consults it.

Scope

• Resolve the encoder at runtime from an ordered preference list (NVENC / AMF / QSV / VideoToolbox per the host GPU, then libx264 last).
• Try each candidate in order and keep the first that actually opens, rather than trusting the capability list alone — fix(export): prove the encoder works instead of inferring it from -encoders established that lesson on the Electron side.
• Log the encoder actually selected, so a support report says which one ran.
• No user-facing selector. Selection is automatic, derived from the host's hardware.

Out of scope

• The preferSoftwareEncoder override — separate PR.
• WARP fallback for D3D device creation — separate PR.

Acceptance

• Native export succeeds on a host with no AMF encoder available.
• The chosen encoder is observable in the logs.
• Unit coverage for the ordering and for the fallback to libx264.

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