feat(recovery): clean mid-stream loss recovery — freeze-until-reanchor + AMD LTR-RFI

Removes the "gray frames with motion" artifact on Vulkan-Video clients and lets
AMD/NVENC hosts re-anchor after loss WITHOUT a 20-40x IDR spike.

Client (pf-client-core): after a reference loss the hardware decoder conceals the
missing-reference deltas (on RADV, a gray plate with new motion painted over) and
returns Ok. The pump now freezes on the last good picture until a clean re-anchor
instead of showing the concealment — lifting on a real IDR, an intra-refresh
recovery mark (2nd wave boundary), or an LTR-RFI recovery anchor (1st). The
frame_index gap is the early, precise loss signal and drives an RFI request.

Host recovery signals (inert unless the backend supports them):
- USER_FLAG_RECOVERY_POINT — intra-refresh wave boundary (NVENC constrained GDR).
- USER_FLAG_RECOVERY_ANCHOR — AMD LTR reference-frame-invalidation recovery frame.

AMD LTR-RFI (encode/windows/amf.rs) — the AMD twin of NVENC RFI. AMF's AVC/HEVC API
has no constrained-intra property (intra-refresh cannot heal; PSNR-proven), so the
only clean-recovery lever is user LTR: mark frames as long-term references, and on
loss force the next frame to re-reference the newest known-good one — a clean
P-frame, not an IDR. Two rotating LTR slots, ~0.5s mark cadence, on by default for
AVC/HEVC (PUNKTFUNK_NO_AMF_LTR disables). invalidate_ref_frames picks the newest LTR
before the loss; a range older than the live slots falls back to a keyframe.

Protocol (punktfunk-core): RfiRequest control message + NativeClient::request_rfi().
Host: RfiRequest dispatch -> invalidate_ref_frames (IDR fallback); an RFI success
anchors the keyframe cooldown so the client's frames_dropped echo of the same loss
is coalesced away rather than emitting a redundant IDR.

Spike: synthetic NV12 GPU source for headless AMF encoder testing.

Validated: core rfi_request_roundtrip; pf-client-core 31 unit tests
(incl. an_rfi_anchor_lifts_immediately); punktfunk-host builds + 271 tests on Linux;
punktfunk-host builds clean on Windows; real AMD iGPU spike (invalidate at frame 90
forced re-reference to LTR frame 60 — 180 frames, keyframes=1, no recovery IDR).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/punktfunk-host/src/spike.rs

@@ -22,6 +22,11 @@ use std::time::Instant;
 pub enum Source {
     /// Deterministic moving BGRx test pattern — no capture session required.
     Synthetic,
+    /// Deterministic moving NV12 texture on the GPU (Windows only) — no capture session required.
+    /// Feeds the native AMF / D3D11 zero-copy encoders, which demand an NV12 GPU texture the CPU
+    /// `Synthetic` source can't give them. Used to validate GPU-encoder behaviour (e.g. AMF
+    /// intra-refresh) headlessly.
+    SyntheticNv12,
     /// Live monitor via the xdg ScreenCast portal + PipeWire.
     Portal,
     /// KWin virtual output created at `width`x`height` (zkde_screencast). Lets us validate
@@ -56,6 +61,27 @@ pub fn run(opts: Options) -> Result<()> {
             );
             Box::new(SyntheticCapturer::new(opts.width, opts.height, opts.fps))
         }
+        Source::SyntheticNv12 => {
+            #[cfg(target_os = "windows")]
+            {
+                tracing::info!(
+                    width = opts.width,
+                    height = opts.height,
+                    fps = opts.fps,
+                    "spike source: synthetic NV12 GPU texture (moving luma ramp)"
+                );
+                Box::new(
+                    capture::synthetic_nv12::SyntheticNv12Capturer::new(
+                        opts.width, opts.height, opts.fps,
+                    )
+                    .context("open synthetic NV12 capturer")?,
+                )
+            }
+            #[cfg(not(target_os = "windows"))]
+            {
+                anyhow::bail!("--source synthetic-nv12 is Windows-only (native AMF / D3D11 encoders)");
+            }
+        }
         Source::Portal => {
             tracing::info!("spike source: xdg ScreenCast portal (live monitor)");
             capture::open_portal_monitor().context("open portal capturer")?