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/windows/nvenc.rs

@@ -13,7 +13,7 @@
 //! Needs a real NVIDIA GPU at runtime (session creation fails otherwise) — compiles GPU-less, but
 //! `open`/`submit` only succeed on a GPU box. The software encoder (`super::sw`) is the fallback.
 
-use super::{Codec, EncodedFrame, Encoder};
+use super::{Codec, EncodedFrame, Encoder, EncoderCaps};
 use crate::capture::{CapturedFrame, FramePayload, PixelFormat};
 use anyhow::{anyhow, bail, Context, Result};
 use std::collections::{HashMap, VecDeque};
@@ -732,6 +732,15 @@ impl Encoder for NvencD3d11Encoder {
         self.force_kf = true;
     }
 
+    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.
+        EncoderCaps {
+            supports_rfi: self.rfi_supported,
+            supports_hdr_metadata: self.hdr,
+        }
+    }
+
     fn set_hdr_meta(&mut self, meta: Option<punktfunk_core::quic::HdrMeta>) {
         // Stored and emitted as in-band SEI on the next keyframe (see `submit`). Cheap to call every
         // frame; only changes when the source is regraded or HDR toggles.