ci+fixtures: UCRT64 msys2 env; fix remaining fixture paths

ci (Windows): the runner's msys2 shell starts as the base MSYS
environment (MSYSTEM=MSYS), which install-deps.sh rejects. Set the
job-level MSYSTEM=UCRT64 env and prepend /ucrt64/bin to PATH in every
Windows step, so pacman installs and the toolchain resolve against the
mingw-w64-ucrt-x86_64 packages.

fixtures: the restructured workspace moved the media fixtures into
crates/oak-app/tests; the remaining references pointed at the old
repo-root tests/ — oak-codec realmedia_tests + hwdecode, oak-node
serializer golden, oak-worker procpool integration. All point at
../oak-app/tests now (oak-app's own tests/demo.mp4 references were
already correct after the move).

.gitea/workflows/ci.yml

@@ -182,6 +182,10 @@ jobs:
   windows:
     name: Build & test (Windows)
     runs-on: oak-windows-2025
+    env:
+      # The runner's msys2 shell starts as the base MSYS environment; the
+      # build needs the UCRT64 one (mingw-w64-ucrt-x86_64-* toolchain).
+      MSYSTEM: UCRT64
     defaults:
       run:
         shell: msys2 {0}
@@ -198,6 +202,7 @@ jobs:
       - name: Install system dependencies
         run: |
           cd "$GITHUB_WORKSPACE"
+          export PATH="/ucrt64/bin:$PATH"
           bash ./tooling/install-deps.sh
           pacman -S --needed --noconfirm \
             mingw-w64-ucrt-x86_64-cmake \
@@ -212,6 +217,7 @@ jobs:
       - name: Configure build environment
         run: |
           cd "$GITHUB_WORKSPACE"
+          export PATH="/ucrt64/bin:$PATH"
           echo "OCIO_RS_ENABLE_REAL=1" >> "$GITHUB_ENV"
           echo "OCIO_INSTALL_DIR=/ucrt64" >> "$GITHUB_ENV"
           echo "OCIO_RS_LINK=dynamic" >> "$GITHUB_ENV"
@@ -260,6 +266,7 @@ jobs:
       - name: Build project FFmpeg
         run: |
           cd "$GITHUB_WORKSPACE"
+          export PATH="/ucrt64/bin:$PATH"
           bash tooling/ffmpeg/build-ffmpeg.sh
           echo "FFMPEG_DIR=$(cygpath -m "$PWD/.cache/ffmpeg")" >> "$GITHUB_ENV"
 
@@ -269,6 +276,7 @@ jobs:
       - name: Build
         run: |
           cd "$GITHUB_WORKSPACE"
+          export PATH="/ucrt64/bin:$PATH"
           # The runner's job hook injects the MSVC INCLUDE/LIB into every
           # step; clear them in-step (they poison the MinGW compiles with
           # MSVC SDK headers).
@@ -284,6 +292,7 @@ jobs:
       - name: Test
         run: |
           cd "$GITHUB_WORKSPACE"
+          export PATH="/ucrt64/bin:$PATH"
           unset INCLUDE LIB
           # See Build (Windows): trailing -lmsvcrt for the mingw-w64
           # _assert/__msvcrt_assert link-order breakage.