feat(host/linux): default the tiled zero-copy path to GPU NV12 (NVENC fed native YUV)

A/B'd on the Bazzite box (RTX 5070 Ti, KWin 6.6, driver 595, 1080p60
over the LAN): pixel-correct decode (full desktop, no tint/banding),
latency-neutral idle (p50 1.47ms RGB vs 1.52ms NV12, both 2400/2400
frames), CPU-neutral — and it deletes NVENC's internal RGB->YUV CSC
from the SM/3D engine a game saturates (video 40%+SM 15% -> video
26%+SM 2% measured on Windows). Matches the Windows host default.
PUNKTFUNK_NV12=0 restores the RGB feed; LINEAR/gamescope captures are
unaffected.

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

crates/punktfunk-host/src/linux/zerocopy/mod.rs

@@ -63,12 +63,14 @@ pub fn enabled() -> bool {
     }
 }
 
-/// Whether the NV12 convert path is opted in (`PUNKTFUNK_NV12` truthy). When set AND the zero-copy
-/// tiled-GL path is active, the capturer produces native NV12 (BT.709 limited range) on the GPU and
-/// feeds NVENC YUV directly — deleting NVENC's internal RGB→YUV CSC (Tier 2A). Off by default: the
-/// existing RGB/BGRx path is then 100% unchanged.
+/// Whether the tiled-GL zero-copy path converts to NV12 on the GPU and feeds NVENC native YUV —
+/// deleting NVENC's internal RGB→YUV CSC, which otherwise runs on the SM/3D engine the game
+/// saturates (Tier 2A). **Default ON** (validated color-correct on the RTX 5070 Ti via
+/// `nv12-selftest` + live decode on dev + Bazzite/KWin boxes; latency- and CPU-neutral idle,
+/// frees SM headroom under load — the same default the Windows host ships). `PUNKTFUNK_NV12=0`
+/// restores the RGB/BGRx feed. LINEAR (gamescope/Vulkan-bridge) captures are unaffected either way.
 pub fn nv12_enabled() -> bool {
-    flag("PUNKTFUNK_NV12")
+    flag_opt("PUNKTFUNK_NV12").unwrap_or(true)
 }
 
 /// DRM FourCC for a packed 32-bit format name (little-endian, e.g. `b"XR24"`).