fix(video): honor the signaled CSC matrix end-to-end + tvOS HDR presentation

Clients derive Y'CbCr->RGB from the stream's SIGNALED matrix x range x depth
via shared csc rows (Rust csc_rows + Swift CscRows) instead of hardcoded
709/2020 - a BT.601-signaled stream (a Linux host's RGB-input NVENC) no longer
renders with a constant hue error. Host-side signaling made honest across
NVENC/VAAPI/openh264/GameStream and the session plan's chroma/bit-depth.
Decoded color-bar fixtures (601/709 x limited/full) pin the math in tests on
both cores.

Same presenter, tvOS HDR: tvOS has no Metal EDR API and a bare PQ colorspace
tag composites UNTONE-MAPPED (the "overblown" Apple TV report), so HDR now
splits on the display's live EDR headroom - PQ passthrough when the
per-session AVDisplayManager mode switch landed (a real HDR10 output
tone-maps itself), else an in-shader PQ->SDR tone-map (203-nit reference
white, extended-Reinhard 1000-nit knee, 2020->709) into the proven SDR layer
config. The 10-bit stream keeps its full decode depth either way.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

crates/punktfunk-host/src/session_plan.rs

@@ -132,6 +132,16 @@ impl SessionPlan {
         let gpu = {
             let force_cpu_for_nvenc_444 =
                 self.chroma.is_444() && !crate::encode::linux_zero_copy_is_vaapi();
+            if gpu && force_cpu_for_nvenc_444 {
+                // Surface the trade loudly: this is the single biggest per-frame cost a 4:4:4
+                // session adds (full-res CPU readback + swscale RGB→YUV444P every frame), and
+                // it looks like an unexplained fps ceiling if you don't know it happened.
+                tracing::warn!(
+                    "4:4:4 session on the NVENC path: zero-copy GPU capture DISABLED — every \
+                     frame is CPU RGB + swscale RGB→YUV444P; expect a lower fps ceiling than \
+                     4:2:0 at this mode"
+                );
+            }
             gpu && !force_cpu_for_nvenc_444
         };
         crate::capture::OutputFormat {