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.

vcpkg.json

@@ -0,0 +1,35 @@
+{
+	"name": "oak",
+	"version": "0.5.0",
+	"comment": "Windows dependencies (CI/CD, MSVC): the project's FFmpeg with every free codec + hwaccel (mirrors tooling/ffmpeg/build-ffmpeg.sh's configure), plus pkgconf so the -sys crates can find the .pc files. ffmpeg version pinning via builtin-baseline is a follow-up once CI reports the resolved version (see .github/workflows/ci.yml).",
+	"dependencies": [
+		{
+			"name": "ffmpeg",
+			"features": [
+				"gpl",
+				"version3",
+				"x264",
+				"x265",
+				"dav1d",
+				"vpx",
+				"openh264",
+				"openjpeg",
+				"theora",
+				"webp",
+				"mp3lame",
+				"opus",
+				"vorbis",
+				"speex",
+				"snappy",
+				"ass",
+				"freetype",
+				"fribidi",
+				"fontconfig",
+				"gnutls",
+				"ffnvcodec"
+			]
+		},
+		"pkgconf",
+		"librsvg"
+	]
+}