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/amf.rs

@@ -2014,9 +2014,6 @@ impl Encoder for AmfEncoder {
             // frame, force a later one to re-reference it). True only when the live driver accepted
             // the LTR slots at open — otherwise loss recovery falls back to a full IDR.
             supports_rfi: self.ltr_active,
-            // In-band mastering/CLL via `*InHDRMetadata` (HEVC SEI / AV1 metadata OBU); AVC has
-            // no such property (and no HDR sessions negotiate H.264).
-            supports_hdr_metadata: self.ten_bit && self.props.hdr_metadata.is_some(),
             // Permanent: VCN hardware does not encode 4:4:4.
             chroma_444: false,
             // True only when `PUNKTFUNK_INTRA_REFRESH` asked for the wave AND the live driver
@@ -2715,10 +2712,6 @@ mod tests {
             }
         };
         enc.set_hdr_meta(Some(sample_hdr_meta()));
-        assert!(
-            enc.caps().supports_hdr_metadata,
-            "HEVC 10-bit reports HDR SEI capability"
-        );
         let mut aus: Vec<EncodedFrame> = Vec::new();
         for i in 0..6 {
             let frame = CapturedFrame {

crates/pf-encode/src/enc/windows/nvenc.rs

@@ -1690,17 +1690,14 @@ impl Encoder for NvencD3d11Encoder {
     }
 
     fn caps(&self) -> EncoderCaps {
-        // RFI is probed once at open (`rfi_supported`); HDR SEI rides keyframes whenever the
-        // session is in HDR mode. Both are the real capabilities the session glue routes on.
+        // RFI is probed once at open (`rfi_supported`) — the real capability the session glue
+        // routes on. (In-band HDR SEI needs no cap: it rides keyframes on HEVC/H.264 HDR
+        // sessions — see `submit` — and every first-party client reads the grade out-of-band
+        // via the 0xCE datagram regardless.)
         EncoderCaps {
             // The Windows capture path composites the pointer; this backend never reads `frame.cursor`.
             blends_cursor: false,
             supports_rfi: self.rfi_supported,
-            // In-band mastering/CLL is attached as keyframe SEI on HEVC/H.264 only — AV1 carries
-            // it in METADATA OBUs (`HDR_MDCV`/`HDR_CLL`), which this backend doesn't emit yet
-            // (see `submit`); the grade still reaches punktfunk clients out-of-band via the 0xCE
-            // datagram. Don't claim a capability the AV1 path doesn't have.
-            supports_hdr_metadata: self.hdr && self.codec != Codec::Av1,
             // Reflects what the session actually configured (cleared in `query_caps` if the GPU lacks
             // YUV444 encode), so the glue can confirm 4:4:4 vs the negotiated request.
             chroma_444: self.chroma_444,

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.