feat(windows-host): EncoderCaps — query RFI/HDR-SEI caps (Goal-1 stage 5, tightening 3)

The last §2.3 seam-trait tightening: give `Encoder` a `caps() -> EncoderCaps`
so the session glue routes by *query* instead of relying on the no-op/`false`
defaults of `invalidate_ref_frames`/`set_hdr_meta`.

`EncoderCaps { supports_rfi, supports_hdr_metadata }` is a cheap `Copy` struct.
The trait gains a default `caps()` returning `EncoderCaps::default()` (all
false) — correct for every SDR/libavcodec backend (Linux NVENC, VAAPI, AMF/QSV,
software openh264), so they need no change. Only the Windows direct-NVENC path
(`NvencD3d11Encoder`) overrides it, reporting the real `rfi_supported` (probed
once at open via `nvEncGetEncodeCaps`) and `hdr` (HDR-SEI on keyframes).

Consumer: the GameStream encode loop (`gamestream/stream.rs`) hoists
`supports_rfi` once before the loop and gates the loss-recovery path on it —
`!(supports_rfi && enc.invalidate_ref_frames(..))` forces a keyframe directly
on non-RFI encoders instead of making an always-`false` call every loss event.
Behaviour-preserving (same keyframe/RFI outcome), one fewer no-op call, intent
explicit. The native host (punktfunk1) uses FEC+keyframes, no RFI consumer.

Linux `cargo clippy -p punktfunk-host --all-targets -D warnings` clean; the
three edited files are rustfmt-clean. The NVENC override is Windows-only
(1:1 with the existing impl style) → CI/on-glass gate.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/punktfunk-host/src/encode.rs

@@ -71,9 +71,34 @@ impl Codec {
     }
 }
 
+/// Static capabilities an [`Encoder`] declares so the session glue routes loss-recovery and HDR
+/// plumbing by *query* rather than relying on a method's no-op/`false` default. Cheap `Copy`; fixed
+/// for the session (an HDR toggle re-initialises the encoder — re-query if that matters).
+#[derive(Clone, Copy, Debug, Default, PartialEq, Eq)]
+pub struct EncoderCaps {
+    /// The encoder can perform real reference-frame invalidation — i.e.
+    /// [`invalidate_ref_frames`](Encoder::invalidate_ref_frames) can return `true`. When `false`
+    /// the caller skips that always-`false` call and forces a keyframe directly on loss recovery.
+    /// Only the Windows direct-NVENC path implements RFI; libavcodec (Linux NVENC), VAAPI and
+    /// AMF/QSV always keyframe.
+    pub supports_rfi: bool,
+    /// The encoder emits in-band HDR mastering/CLL SEI from [`set_hdr_meta`](Encoder::set_hdr_meta).
+    /// When `false`, `set_hdr_meta` is a no-op and no in-band grade reaches the client. Only the
+    /// Windows direct-NVENC path attaches it today.
+    pub supports_hdr_metadata: bool,
+}
+
 /// A hardware encoder. One per session; runs on the encode thread.
 pub trait Encoder: Send {
     fn submit(&mut self, frame: &CapturedFrame) -> Result<()>;
+    /// This encoder's static [capabilities](EncoderCaps) (RFI, HDR SEI), so the session glue can
+    /// route by query rather than rely on the no-op/`false` defaults of
+    /// [`invalidate_ref_frames`](Self::invalidate_ref_frames) / [`set_hdr_meta`](Self::set_hdr_meta).
+    /// Default: no optional capabilities (the SDR / libavcodec backends) — only the direct-NVENC
+    /// path overrides it.
+    fn caps(&self) -> EncoderCaps {
+        EncoderCaps::default()
+    }
     /// Force the next submitted frame to be an IDR keyframe (e.g. after a client
     /// reference-frame-invalidation request). Default: no-op.
     fn request_keyframe(&mut self) {}