feat(encode): make cursor blending a queryable capability, not an assumption

`open_video`'s `cursor_blend` argument was a request with no answer: lib.rs did
`let _ = cursor_blend;` and only three backends ever read `CapturedFrame::cursor`.
So a session could ask for a composited pointer, get a backend that silently
discards it, and stream with no mouse cursor and nothing in the logs. Two
separately-confirmed audit findings — the VAAPI dmabuf path and the libav-NVENC CUDA
path — are symptoms of that one hole.

`EncoderCaps::blends_cursor` makes it a fact each backend states. The four exhaustive
`EncoderCaps { .. }` constructors mean adding the field is a compile error until every
backend answers, which is the enforcement mechanism for future backends rather than a
side effect. Vulkan Video answers from its ACTUAL configured source rather than
statically: only the CSC path composites (`prep_cursor` feeds the compute shader),
while the RGB-direct/EFC front-end and the native-NV12 source have no compositing
stage at all and merely warn once that the pointer is being dropped.

`open_video` warns when a session asked for blending and the opened backend cannot
deliver it. A warning is deliberately all it does: `open_video` cannot re-plan
capture, so refusing would trade a missing pointer for a dead session. The host owns
`plan.cursor_blend` and is the only layer that can fall back to capturer-side
compositing — this gives it something to base that on.

Enforcement is NOT included. The reviewed design proposed refusing the client's
host-composite flip to keep the client drawing its own pointer, but `CursorRenderMode`
is client->host only: there is no host->client counterpart, so refusing yields no
pointer at all — the same failure it claimed to prevent.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

crates/pf-encode/src/lib.rs

@@ -177,6 +177,23 @@ pub fn open_video(
             },
         }
     };
+    // The session asked for a composited pointer; say so loudly if the backend that actually opened
+    // cannot deliver one. `cursor_blend` was a REQUEST with no answer for most of this crate's life
+    // (`let _ = cursor_blend;` below), and the result was a stream with no mouse cursor and nothing
+    // in the logs — confirmed on the VAAPI dmabuf path and the libav-NVENC CUDA path.
+    //
+    // A warning is deliberately all this does. `open_video` cannot re-plan capture, so refusing here
+    // would only trade a missing pointer for a dead session; the host owns `plan.cursor_blend` and is
+    // the only layer that can fall back to capturer-side compositing. This makes the condition
+    // visible and queryable (`EncoderCaps::blends_cursor`) so that decision can be made upstream.
+    if cursor_blend && !inner.caps().blends_cursor {
+        tracing::warn!(
+            backend,
+            "session negotiated a composited cursor but this encode backend does not blend \
+             CapturedFrame::cursor — the pointer will be MISSING from the stream unless the \
+             capturer composites it"
+        );
+    }
     Ok(Box::new(TrackedEncoder {
         inner,
         _session: pf_gpu::session_begin(gpu),