feat(windows-host): OutputFormat into the capturer — kill the dxgi back-reference (Goal-1 stage 5, tightening 1)

The headline §2.3 seam tightening (the explicit Stage-3 deferral; §5's "highest-severity
coupling"): the capturer is now TOLD its output format instead of re-deriving the encode backend.

New `capture::OutputFormat { gpu, hdr }`, resolved once per session and passed INTO
capture_virtual_output:
  * native punktfunk/1 path: `SessionPlan::output_format()` (gpu = encoder.is_gpu(), from the
    already-resolved plan.encoder — no second probe; hdr = plan.hdr).
  * GameStream + spike paths: `OutputFormat::resolve(hdr)` (gpu from the single `gpu_encode()`
    source, which maps windows_resolved_backend()).

`capture/dxgi.rs DuplCapturer::open` takes `gpu` in and its internal
`!matches!(windows_resolved_backend(), Software)` recompute is DELETED — the capture layer no
longer re-calls the encode layer (the back-reference that could let capture and encode disagree
on whether frames are GPU-resident, plan §2.3/§5). The relay's secure-desktop DDA passes
`gpu_encode()` likewise.

Behavior-preserving: the `gpu` passed in equals the value the capturer used to compute (same
encode-backend resolution). The DDA opens keep `want_hdr=false` (the SDR fallback, unchanged).

Tightenings 2 (HDR/release -> VirtualLease) and 3 (EncoderCaps) split off: (2) needs the
monitor-generation carried on the lease + the keepalive becoming Box<dyn VirtualLease> — that's
the §2.5 ownership-model change (CURRENT_MON_GEN / sudovda::wait_for_monitor_released), so it
moves there; (3) is a small additive follow-on. Documented in the plan.

Verified: Linux cargo check + clippy (-D warnings) + fmt clean. Box build to follow.

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

crates/punktfunk-host/src/session_plan.rs

@@ -86,6 +86,14 @@ pub enum EncoderBackend {
     Software,
 }
 
+impl EncoderBackend {
+    /// True if this backend encodes on the GPU (so the capturer should produce GPU-resident frames). Only
+    /// the software encoder takes CPU staging; `PlatformAuto` (Linux NVENC/VAAPI) is always GPU.
+    pub fn is_gpu(self) -> bool {
+        !matches!(self, EncoderBackend::Software)
+    }
+}
+
 /// The per-session decision, resolved once. `Copy` so it threads through the capture/encode chain
 /// without ceremony (stage 4 folds it, with the rest of the arg soup, into a `SessionContext`).
 #[derive(Clone, Copy, Debug)]
@@ -111,6 +119,16 @@ impl SessionPlan {
             hdr: bit_depth >= 10,
         }
     }
+
+    /// The capturer's target output format (Goal-1 stage 5): `gpu` from the already-resolved `encoder`
+    /// (no second backend probe), `hdr` from the plan. Handed into `capture::capture_virtual_output` so the
+    /// capturer never re-derives the encode backend.
+    pub fn output_format(&self) -> crate::capture::OutputFormat {
+        crate::capture::OutputFormat {
+            gpu: self.encoder.is_gpu(),
+            hdr: self.hdr,
+        }
+    }
 }
 
 /// Process topology. On Windows this is the former `punktfunk1::should_use_helper` logic verbatim; on