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/run-windows.sh

@@ -0,0 +1,61 @@
+#!/usr/bin/env bash
+# run-windows.sh — Cross-compile WoWee for Windows (x86_64) inside a Docker container.
+#
+# Usage (run from project root):
+#   ./container/run-windows.sh [--rebuild-image]
+#
+# Environment variables:
+#   WOWEE_FFX_SDK_REPO  — FidelityFX SDK git repo URL (passed through to container)
+#   WOWEE_FFX_SDK_REF   — FidelityFX SDK git ref / tag      (passed through to container)
+#   REBUILD_IMAGE       — Set to 1 to force a fresh docker build (same as --rebuild-image)
+#
+# Toolchain: LLVM-MinGW (Clang + LLD) targeting x86_64-w64-mingw32-ucrt
+# vcpkg triplet: x64-mingw-static
+
+set -euo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+PROJECT_ROOT="$(cd "${SCRIPT_DIR}/.." && pwd)"
+
+IMAGE_NAME="wowee-builder-windows"
+BUILD_OUTPUT="${PROJECT_ROOT}/build/windows"
+
+# Parse arguments
+REBUILD_IMAGE="${REBUILD_IMAGE:-0}"
+for arg in "$@"; do
+  case "$arg" in
+    --rebuild-image) REBUILD_IMAGE=1 ;;
+    *) echo "Unknown argument: $arg" >&2; exit 1 ;;
+  esac
+done
+
+# Verify Docker is available
+if ! command -v docker &>/dev/null; then
+  echo "Error: docker is not installed or not in PATH." >&2
+  exit 1
+fi
+
+# Build the image (skip if already present and --rebuild-image not given)
+if [[ "$REBUILD_IMAGE" == "1" ]] || ! docker image inspect "$IMAGE_NAME" &>/dev/null; then
+  echo "==> Building Docker image: ${IMAGE_NAME}"
+  docker build \
+    -f "${SCRIPT_DIR}/builder-windows.Dockerfile" \
+    -t "$IMAGE_NAME" \
+    "${SCRIPT_DIR}"
+else
+  echo "==> Using existing Docker image: ${IMAGE_NAME}"
+fi
+
+# Create output directory on the host
+mkdir -p "$BUILD_OUTPUT"
+
+echo "==> Starting Windows cross-compile build (output: ${BUILD_OUTPUT})"
+
+docker run --rm \
+  --mount "type=bind,src=${PROJECT_ROOT},dst=/src,readonly" \
+  --mount "type=bind,src=${BUILD_OUTPUT},dst=/out" \
+  ${WOWEE_FFX_SDK_REPO:+--env "WOWEE_FFX_SDK_REPO=${WOWEE_FFX_SDK_REPO}"} \
+  ${WOWEE_FFX_SDK_REF:+--env "WOWEE_FFX_SDK_REF=${WOWEE_FFX_SDK_REF}"} \
+  "$IMAGE_NAME"
+
+echo "==> Windows cross-compile build complete. Artifacts in: ${BUILD_OUTPUT}"