feat: add multi-platform Docker build system for Linux, macOS, and Windows

Replace the single Ubuntu-based container build with a dedicated
Dockerfile, build script, and launcher for each target platform.

Infrastructure:
- Add .dockerignore to minimize Docker build context
- Add container/builder-linux.Dockerfile (Ubuntu 24.04, GCC, native build)
- Add container/builder-macos.Dockerfile (multi-stage: SDK fetcher + osxcross/Clang 18)
- Add container/builder-windows.Dockerfile (LLVM-MinGW 20240619, vcpkg)
- Add container/macos/sdk-fetcher.py (auto-fetch macOS SDK from Apple catalog)
- Add container/macos/osxcross-toolchain.cmake (auto-detecting CMake toolchain)
- Add container/macos/triplets/arm64-osx-cross.cmake
- Add container/macos/triplets/x64-osx-cross.cmake
- Remove container/builder-ubuntu.Dockerfile (replaced by per-platform Dockerfiles)
- Remove container/build-in-container.sh and container/build-wowee.sh (replaced)

Build scripts (run inside containers):
- Add container/build-linux.sh (tar copy, FidelityFX clone, cmake/ninja)
- Add container/build-macos.sh (arch detection, vcpkg triplet, cross-compile)
- Add container/build-windows.sh (Vulkan import lib via dlltool, cross-compile)

Launcher scripts (run on host):
- Add container/run-linux.sh, run-macos.sh, run-windows.sh (bash)
- Add container/run-linux.ps1, run-macos.ps1, run-windows.ps1 (PowerShell)

Documentation:
- Add container/README.md (quick start, options, file structure, troubleshooting)
- Add container/FLOW.md (comprehensive build flow for each platform)

CMake changes:
- Add macOS cross-compile support (VulkanHeaders, -undefined dynamic_lookup)
- Add LLVM-MinGW/Windows cross-compile support
- Detect osxcross toolchain and vcpkg triplets

Other:
- Update vcpkg.json with ffmpeg feature flags
- Update resources/wowee.rc version string

container/builder-macos.Dockerfile

@@ -0,0 +1,143 @@
+FROM ubuntu:24.04 AS sdk-fetcher
+
+# Stage 1: Fetch macOS SDK from Apple's public software update catalog.
+# This avoids requiring the user to supply the SDK tarball manually.
+# The SDK is downloaded, extracted, and packaged as a .tar.gz.
+
+ENV DEBIAN_FRONTEND=noninteractive
+
+RUN apt-get update && \
+    apt-get install -y --no-install-recommends \
+        ca-certificates \
+        python3 \
+        cpio \
+        tar \
+        gzip \
+        xz-utils && \
+    rm -rf /var/lib/apt/lists/*
+
+COPY macos/sdk-fetcher.py /opt/sdk-fetcher.py
+RUN python3 /opt/sdk-fetcher.py /opt/sdk
+
+# ---------------------------------------------------------------------------
+
+FROM ubuntu:24.04 AS builder
+
+# Stage 2: macOS cross-compile image using osxcross + Clang 18.
+#
+# Target triplets (auto-detected from osxcross):
+#   arm64-apple-darwinNN  (Apple Silicon)
+#   x86_64-apple-darwinNN (Intel)
+# Default: arm64. Override with MACOS_ARCH=x86_64 env var at run time.
+
+ENV DEBIAN_FRONTEND=noninteractive
+ENV OSXCROSS_VERSION=1.5
+
+RUN apt-get update && \
+    apt-get install -y --no-install-recommends \
+        ca-certificates \
+        cmake \
+        ninja-build \
+        git \
+        python3 \
+        curl \
+        wget \
+        xz-utils \
+        zip \
+        unzip \
+        tar \
+        make \
+        patch \
+        libssl-dev \
+        zlib1g-dev \
+        pkg-config \
+        libbz2-dev \
+        libxml2-dev \
+        libz-dev \
+        liblzma-dev \
+        uuid-dev \
+        python3-lxml \
+        gnupg \
+        software-properties-common && \
+    wget -qO- https://apt.llvm.org/llvm-snapshot.gpg.key | apt-key add - && \
+    echo "deb http://apt.llvm.org/jammy/ llvm-toolchain-jammy-18 main" > /etc/apt/sources.list.d/llvm-18.list && \
+    apt-get update && \
+    apt-get install -y --no-install-recommends \
+        clang-18 \
+        lld-18 \
+        llvm-18 && \
+    ln -sf /usr/bin/clang-18 /usr/bin/clang && \
+    ln -sf /usr/bin/clang++-18 /usr/bin/clang++ && \
+    ln -sf /usr/bin/lld-18 /usr/bin/lld && \
+    ln -sf /usr/bin/ld.lld-18 /usr/bin/ld.lld && \
+    ln -sf /usr/bin/llvm-ar-18 /usr/bin/llvm-ar && \
+    rm -rf /var/lib/apt/lists/*
+
+# Build osxcross with SDK from stage 1
+RUN git clone --depth 1 https://github.com/tpoechtrager/osxcross.git /opt/osxcross
+
+COPY --from=sdk-fetcher /opt/sdk/ /opt/osxcross/tarballs/
+
+ENV MACOSX_DEPLOYMENT_TARGET=13.0
+RUN cd /opt/osxcross && \
+    unset OSXCROSS_VERSION && \
+    UNATTENDED=1 ./build.sh && \
+    rm -rf /opt/osxcross/build /opt/osxcross/tarballs
+
+ENV PATH="/opt/osxcross/target/bin:${PATH}"
+ENV OSXCROSS_TARGET_DIR="/opt/osxcross/target"
+ENV MACOSX_DEPLOYMENT_TARGET=13.0
+
+# Create unprefixed symlinks for macOS tools that vcpkg/CMake expect
+RUN cd /opt/osxcross/target/bin && \
+    for tool in install_name_tool otool lipo codesign; do \
+      src="$(ls *-apple-darwin*-"${tool}" 2>/dev/null | head -1)"; \
+      if [ -n "$src" ]; then \
+        ln -sf "$src" "$tool"; \
+      fi; \
+    done
+
+# Custom osxcross toolchain + vcpkg triplets
+COPY macos/osxcross-toolchain.cmake /opt/osxcross-toolchain.cmake
+COPY macos/triplets/ /opt/vcpkg-triplets/
+
+# Extra tools needed by vcpkg's Mach-O rpath fixup and ffmpeg x86 asm
+RUN apt-get update && \
+    apt-get install -y --no-install-recommends file nasm && \
+    rm -rf /var/lib/apt/lists/*
+
+# vcpkg — macOS cross triplets (arm64-osx-cross / x64-osx-cross)
+ENV VCPKG_ROOT=/opt/vcpkg
+RUN git clone --depth 1 https://github.com/microsoft/vcpkg.git "${VCPKG_ROOT}" && \
+    "${VCPKG_ROOT}/bootstrap-vcpkg.sh" -disableMetrics
+
+# Pre-install deps for both arches; the launcher script picks the right one at run time.
+RUN "${VCPKG_ROOT}/vcpkg" install \
+        sdl2[vulkan] \
+        openssl \
+        glew \
+        glm \
+        zlib \
+        ffmpeg \
+    --triplet arm64-osx-cross \
+    --overlay-triplets=/opt/vcpkg-triplets
+
+RUN "${VCPKG_ROOT}/vcpkg" install \
+        sdl2[vulkan] \
+        openssl \
+        glew \
+        glm \
+        zlib \
+        ffmpeg \
+    --triplet x64-osx-cross \
+    --overlay-triplets=/opt/vcpkg-triplets
+
+# Vulkan SDK headers (MoltenVK is the runtime — headers only needed to compile)
+RUN apt-get update && \
+    apt-get install -y --no-install-recommends libvulkan-dev glslang-tools && \
+    rm -rf /var/lib/apt/lists/*
+
+COPY build-macos.sh /build-platform.sh
+RUN chmod +x /build-platform.sh
+
+ENTRYPOINT ["/build-platform.sh"]