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

.dockerignore

@@ -0,0 +1,50 @@
+# .dockerignore — Exclude files from the Docker build context.
+# Keeps the context small and prevents leaking build artifacts or secrets.
+
+# Build outputs
+build/
+cache/
+
+# Git history
+.git/
+.gitignore
+.github/
+
+# Large external directories (fetched at build time inside the container)
+extern/FidelityFX-FSR2/
+extern/FidelityFX-SDK/
+
+# IDE / editor files
+.vscode/
+.idea/
+*.swp
+*.swo
+*~
+.DS_Store
+
+# Documentation (not needed for build)
+docs/
+*.md
+!container/*.md
+
+# Test / tool outputs
+logs/
+
+# Host build scripts that run outside the container (not needed inside)
+build.sh
+build.bat
+build.ps1
+rebuild.sh
+rebuild.bat
+rebuild.ps1
+clean.sh
+debug_texture.*
+extract_assets.*
+extract_warden_rsa.py
+restart-worldserver.sh
+test.sh
+
+# macOS SDK tarballs that may be temporarily placed here
+*.tar.xz
+*.tar.gz
+*.tar.bz2