refactor(host/W4): make the capture→encode edge one-way (OutputFormat back-ref)

The capture.rs facade no longer re-derives the encode backend. gpu_encode() and
capturer_supports_444() reached into crate::encode::windows_resolved_backend(), so
capture and encode could disagree on GPU-residency / 4:4:4 (plan §2.4). Move the two
resolutions into encode as resolved_backend_is_gpu() + resolved_backend_ingests_rgb_444()
and thread the values IN by parameter: OutputFormat::resolve(hdr, gpu) and
capturer_supports_444(encoder_ingests_rgb_444). Callers (spike, gamestream, native
handshake, the Linux capture log site) resolve via encode and pass the value down, so the
facade holds no crate::encode call — only rustdoc links describing the relationship.
Completes task #8 of W4.

Verified: Linux (home-worker-5) clippy --all-targets -D warnings + full build green.
Windows (.173) verify owed — box was offline this session.

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

crates/punktfunk-host/src/native/handshake.rs

@@ -246,7 +246,8 @@ pub(super) async fn negotiate(
     // today (full-chroma IDD-push capture is a follow-up), so it returns false there and the host
     // negotiates 4:2:0. (Replaces the old `single_process` gate — single-process is now the only
     // topology, and 4:4:4 routed to DDA, which was removed.)
-    let capture_supports_444 = crate::capture::capturer_supports_444();
+    let capture_supports_444 =
+        crate::capture::capturer_supports_444(crate::encode::resolved_backend_ingests_rgb_444());
     // The GPU probe opens a real (tiny) encoder on first use, so run it off the reactor like the
     // compositor probe above (blocking probes → spawn_blocking). Short-circuit so it only runs when
     // the cheap gates already pass. The result is cached process-wide (a negative latches until