ci/cd: Windows goes vcpkg manifest + MSVC on the VS 2026 runner

The MinGW path kept fighting the environment (the GitHub image's
MSVC INCLUDE/LIB poison the GNU compiles; ocio-sys' fork then
dragged the MSVC-only headers into g++ and died on vcruntime.h).
The Windows jobs on both workflows now:

- run on warp-windows-2025-vs2026-x64-16x (preinstalled VS 2026)
  with the stable MSVC Rust toolchain;
- install dependencies through vcpkg MANIFEST mode: vcpkg.json at
  the repo root carries FFmpeg with every free codec + hwaccel
  (mirroring build-ffmpeg.sh's configure), pkgconf and librsvg;
  the vcpkg_installed tree plus the binary-cache archives are
  cached on the manifest hash with save-always;
- build OCIO bundled (ocio-sys' vendored sources are what MSVC
  wants — the MSYS2 package was the workaround, not the
  preference), so OCIO_RS_NO_MSVC_INCLUDES is gone;
- ship the vcpkg runtime DLLs next to the binaries in the NSIS
  installer with a static-CRT release build (no vcruntime DLLs),
  replacing the ntldd-based MSYS2 bundling.

FFmpeg version pinning via builtin-baseline is a documented
follow-up: the Configure step logs vcpkg list so the first green
run reports the resolved versions. docs/build.md keeps the MSYS2
flow as the local alternative and points at the CI path.

docs/build.md

@@ -86,6 +86,12 @@ cargo test  --workspace         # Linux: see "headless tests" below
 
 ## Windows (MSYS2 UCRT64)
 
+> **CI/CD note**: the GitHub Windows CI/CD no longer uses this path — it
+> builds MSVC-ABI on `warp-windows-2025-vs2026-x64-16x` with vcpkg
+> manifest mode (`vcpkg.json` at the repo root: FFmpeg with every free
+> codec + hwaccel, pkgconf, librsvg) and the vendored static OCIO. The
+> MSYS2 flow below remains the documented local-build alternative.
+
 The Windows build targets **x86_64-pc-windows-gnu** with MSYS2's own
 Rust; the MSVC toolchain is not supported (the build scripts emit
 Unix-style link args the MSVC linker rejects).