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.

.gitignore

@@ -15,6 +15,9 @@ CmakeSettings.json
 # clangd's index and likely other things that need not be in the repository
 .cache/
 
+# vcpkg manifest-mode install tree (Windows CI/CD; see vcpkg.json)
+vcpkg_installed/
+
 # Generated packaging assets (icons/icon.png is produced from Oak_Icon.svg
 # by rsvg-convert in the CD workflow; see .github/workflows/cd.yml)
 /icons/