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-macos.ps1

@@ -0,0 +1,71 @@
+# run-macos.ps1 — Cross-compile WoWee for macOS (arm64 or x86_64) inside a Docker container.
+#
+# Usage (run from project root):
+#   .\container\run-macos.ps1 [-RebuildImage] [-Arch arm64|x86_64]
+#
+# The macOS SDK is fetched automatically inside the Docker build from Apple's
+# public software update catalog.  No manual SDK download required.
+#
+# 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)
+
+param(
+    [switch]$RebuildImage,
+    [ValidateSet("arm64", "x86_64")]
+    [string]$Arch = "arm64"
+)
+
+$ErrorActionPreference = "Stop"
+
+$ScriptDir   = Split-Path -Parent $MyInvocation.MyCommand.Definition
+$ProjectRoot = (Resolve-Path "$ScriptDir\..").Path
+
+$ImageName   = "wowee-builder-macos"
+$BuildOutput = "$ProjectRoot\build\macos"
+
+# Verify Docker is available
+if (-not (Get-Command docker -ErrorAction SilentlyContinue)) {
+    Write-Error "docker is not installed or not in PATH."
+    exit 1
+}
+
+# Build the image (skip if already present and -RebuildImage not given)
+$imageExists = docker image inspect $ImageName 2>$null
+if ($RebuildImage -or -not $imageExists) {
+    Write-Host "==> Building Docker image: $ImageName"
+    Write-Host "    (SDK will be fetched automatically from Apple's catalog)"
+    docker build `
+        -f "$ScriptDir\builder-macos.Dockerfile" `
+        -t $ImageName `
+        "$ScriptDir"
+    if ($LASTEXITCODE -ne 0) { exit $LASTEXITCODE }
+} else {
+    Write-Host "==> Using existing Docker image: $ImageName"
+}
+
+# Create output directory on the host
+New-Item -ItemType Directory -Force -Path $BuildOutput | Out-Null
+
+Write-Host "==> Starting macOS cross-compile build (arch=$Arch, output: $BuildOutput)"
+
+$dockerArgs = @(
+    "run", "--rm",
+    "--mount", "type=bind,src=$ProjectRoot,dst=/src,readonly",
+    "--mount", "type=bind,src=$BuildOutput,dst=/out",
+    "--env", "MACOS_ARCH=$Arch"
+)
+
+if ($env:WOWEE_FFX_SDK_REPO) {
+    $dockerArgs += @("--env", "WOWEE_FFX_SDK_REPO=$env:WOWEE_FFX_SDK_REPO")
+}
+if ($env:WOWEE_FFX_SDK_REF) {
+    $dockerArgs += @("--env", "WOWEE_FFX_SDK_REF=$env:WOWEE_FFX_SDK_REF")
+}
+
+$dockerArgs += $ImageName
+
+& docker @dockerArgs
+if ($LASTEXITCODE -ne 0) { exit $LASTEXITCODE }
+
+Write-Host "==> macOS cross-compile build complete. Artifacts in: $BuildOutput"