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/windows/qsv.rs

@@ -1471,9 +1471,6 @@ impl Encoder for QsvEncoder {
             // As Windows NVENC: the capturer composites; this backend never reads `frame.cursor`.
             blends_cursor: false,
             supports_rfi: self.ltr_active,
-            // In-band mastering/CLL at IDR (HEVC prefix SEI / AV1 metadata OBU); AVC sessions
-            // are never HDR.
-            supports_hdr_metadata: self.ten_bit && self.codec != Codec::H264,
             chroma_444: false,
             intra_refresh: self.ir_active,
             // Unvalidated on-glass — the host keeps the IDR recovery path until then.