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/enc/codec.rs

@@ -294,6 +294,19 @@ pub struct EncoderCaps {
     /// `USER_FLAG_RECOVERY_POINT` on every Nth emitted AU, re-phased at each IDR). 0 when intra-refresh
     /// is off. Only consulted when [`intra_refresh_recovery`](Self::intra_refresh_recovery) is set.
     pub intra_refresh_period: u32,
+    /// The encoder composites [`CapturedFrame::cursor`] into the picture it encodes.
+    ///
+    /// `open_video`'s `cursor_blend` argument is a REQUEST, and for most of this crate's life it was
+    /// nothing else: `lib.rs` literally did `let _ = cursor_blend;` and only three backends ever read
+    /// `frame.cursor`. So a session could ask for a composited pointer, get a backend that silently
+    /// discards it, and stream with no mouse cursor at all — the confirmed symptom on the VAAPI
+    /// dmabuf path and the libav-NVENC CUDA path.
+    ///
+    /// This makes the answer queryable instead of assumed. It is deliberately a plain fact about the
+    /// encoder, not a policy: what to DO when a session wants blending and the backend cannot is the
+    /// host's call, since only the host can re-plan capture (fall back to capturer-side compositing).
+    /// `open_video` can only warn, which it does.
+    pub blends_cursor: bool,
 }
 
 /// A hardware encoder. One per session; runs on the encode thread.