fix(encode): delete the write-only EncoderCaps::supports_hdr_metadata

A caps field nothing reads is a contract nobody honors — and this one shipped
write-only: its single reader anywhere in the workspace was a hardware-gated
assertion inside pf-encode's own AMF smoke test. Both planes send the static
HDR grade out-of-band unconditionally (the native 0xCE datagram per keyframe,
the GameStream 0x010e control message), every first-party client reads
exclusively that path, and none parse in-band SEI — so the host decision the
field was reserved for (suppress out-of-band when the encoder embeds) can
never validly exist. The field's doc contract had also rotted in two
directions: it claimed set_hdr_meta no-ops when false (native AMF and QSV
consume it regardless) and that only Windows direct-NVENC attaches in-band
metadata (AMF and QSV do too). The in-band SEI/OBU emission itself is
untouched — it stays a bonus for stock decoders, documented at the emit
sites; the trait docs now describe the real routing.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

crates/pf-encode/src/enc/linux/nvenc_cuda.rs

@@ -1850,7 +1850,6 @@ impl Encoder for NvencCudaEncoder {
             // Composites `frame.cursor` via the SPIR-V blend over the Vulkan-allocated input slot.
             blends_cursor: true,
             supports_rfi: self.rfi_supported,
-            supports_hdr_metadata: self.hdr,
             chroma_444: self.chroma_444,
             intra_refresh: false,
             intra_refresh_recovery: false,