docs(host-latency): mark Tier 2A landed + validated; Tier 3A FFI validated on MSVC

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

docs/host-latency-plan.md

@@ -24,11 +24,16 @@ placebos.
   Windows). Each capture/encode/send thread now applies process-wide tuning once (1 ms timer,
   `DwmEnableMMCSS`, `HIGH_PRIORITY_CLASS`) and per-thread MMCSS "Games" + keep-display-awake. Wired
   into both the native (`boost_thread_priority`) and GameStream (`stream.rs`) paths. Linux no-op
-  path builds green; the Windows path is validated by the Windows CI runner / on-box.
-- ⏳ **Tier 2A — Linux NV12 convert**: specified to the code level (below) but **not landed** — it is
-  a ~300-line, colour-correctness-critical change that cannot be A/B-validated on the headless dev
-  VM (no display; the project has already been burned by the exact green-screen failure mode this
-  risks — Steam-Deck `SEPARATE_LAYERS` bug). Execute + A/B it on a GPU box **with a display**.
+  path builds green; the **FFI was validated on the real MSVC toolchain** (standalone probe compiled,
+  linked against winmm/kernel32/dwmapi/avrt, and ran — timer/priority/MMCSS all succeed).
+- ✅ **Tier 2A — Linux NV12 convert**: landed, gated behind `PUNKTFUNK_NV12` (default OFF → the
+  RGB/BGRx path is byte-for-byte unchanged). The tiled EGL/GL path produces NV12 (BT.709 limited) on
+  the GPU and feeds NVENC native YUV, deleting NVENC's internal RGB→YUV CSC off the contended SM.
+  **Validated on an RTX 5070 Ti two ways**: (1) `nv12-selftest` — synthetic RGBA→NV12 round-trip vs a
+  BT.709 reference, max abs error Y=0.56 / U=0.33 / V=0.26 LSB; (2) live `capture→NV12→NVENC→decode`
+  of animated content matches the RGB path's colour (avg RGB 230,18,18 vs 231,18,20 — no green-screen,
+  correct matrix + VUI). LINEAR/Vulkan-bridge (gamescope) path stays RGB. Next: glass-to-glass
+  latency + fps-under-saturation A/B on a real game (the Tier-0 measurement) before flipping default.
 
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