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/gamestream/stream.rs

@@ -383,7 +383,7 @@ fn open_gs_virtual_source(
     // capturer follows the display). No-op on Linux (8-bit, and `cfg.hdr` is always false there).
     let capturer = capture::capture_virtual_output(
         vout,
-        capture::OutputFormat::resolve(cfg.hdr),
+        capture::OutputFormat::resolve(cfg.hdr, crate::encode::resolved_backend_is_gpu()),
         crate::session_plan::CaptureBackend::resolve(),
     )
     .context("capture virtual output")?;