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/build-linux.sh

@@ -0,0 +1,62 @@
+#!/bin/bash
+# Linux amd64 build entrypoint — runs INSIDE the linux container.
+# Bind-mounts:
+#   /src  (ro) — project source
+#   /out  (rw) — host ./build/linux
+
+set -euo pipefail
+
+SRC=/src
+OUT=/out
+NPROC=$(nproc)
+
+echo "==> [linux] Copying source tree..."
+tar -C "${SRC}" \
+    --exclude='./build' --exclude='./logs' --exclude='./cache' \
+    --exclude='./container' --exclude='./.git' \
+    --exclude='./Data/character' --exclude='./Data/creature' \
+    --exclude='./Data/db' --exclude='./Data/environment' \
+    --exclude='./Data/interface' --exclude='./Data/item' \
+    --exclude='./Data/misc' --exclude='./Data/sound' \
+    --exclude='./Data/spell' --exclude='./Data/terrain' \
+    --exclude='./Data/world' \
+    -cf - . | tar -C /wowee-build-src -xf -
+
+cd /wowee-build-src
+
+echo "==> [linux] Fetching external SDKs (if needed)..."
+if [ ! -f extern/FidelityFX-FSR2/src/ffx-fsr2-api/ffx_fsr2.h ]; then
+    git clone --depth 1 \
+        https://github.com/GPUOpen-Effects/FidelityFX-FSR2.git \
+        extern/FidelityFX-FSR2 || echo "Warning: FSR2 clone failed — continuing without FSR2"
+fi
+
+SDK_REPO="${WOWEE_FFX_SDK_REPO:-https://github.com/Kelsidavis/FidelityFX-SDK.git}"
+SDK_REF="${WOWEE_FFX_SDK_REF:-main}"
+if [ ! -f "extern/FidelityFX-SDK/sdk/include/FidelityFX/host/ffx_frameinterpolation.h" ]; then
+    git clone --depth 1 --branch "${SDK_REF}" "${SDK_REPO}" extern/FidelityFX-SDK \
+        || echo "Warning: FidelityFX-SDK clone failed — continuing without FSR3"
+fi
+
+echo "==> [linux] Configuring with CMake (Release, Ninja, amd64)..."
+cmake -S . -B "${OUT}" \
+    -G Ninja \
+    -DCMAKE_BUILD_TYPE=Release \
+    -DCMAKE_C_COMPILER=gcc \
+    -DCMAKE_CXX_COMPILER=g++ \
+    -DCMAKE_INTERPROCEDURAL_OPTIMIZATION=ON
+
+echo "==> [linux] Building with ${NPROC} cores..."
+cmake --build "${OUT}" --parallel "${NPROC}"
+
+echo "==> [linux] Creating Data symlink..."
+mkdir -p "${OUT}/bin"
+if [ ! -e "${OUT}/bin/Data" ]; then
+    # Relative symlink so it resolves correctly on the host:
+    # build/linux/bin/Data -> ../../../Data (project root)
+    ln -s ../../../Data "${OUT}/bin/Data"
+fi
+
+echo ""
+echo "==> [linux] Build complete. Artifacts in: ./build/linux/"
+echo "    Binary: ./build/linux/bin/wowee"