Frame Studio and Frame Home: cross-platform builds

Frame Studio now builds for Windows (x64, x86, ARM64), Linux (x64, x86, ARM64)
and macOS (Apple Silicon, Intel):
- studio/platform.h with Win32 and POSIX implementations (dialogs through
  tinyfiledialogs on Linux/macOS), a GLFW window beside the Win32 one, and a
  platform-neutral frame pacer
- MoltenVK support in the renderer (portability enumeration) and bundled in
  the macOS app
- 32-bit builds: large-address-aware, LZMA packing within a 32-bit address space
- unaligned file reads in the loaders go through loadUnaligned (no UB)
- scripts/build-studio.sh, studio/build_windows.bat [x64|x86|arm64],
  cmake/llvm-mingw.cmake, and a GitHub Actions workflow building every target

Frame Home installs without Studio: frame/install-frame-home.{sh,ps1} and
"Install Frame Home.bat" (packaged as FrameHome-installer.zip).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

third_party/astcenc/Source/astcenc_mathlib.cpp

@@ -0,0 +1,48 @@
+// SPDX-License-Identifier: Apache-2.0
+// ----------------------------------------------------------------------------
+// Copyright 2011-2021 Arm Limited
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not
+// use this file except in compliance with the License. You may obtain a copy
+// of the License at:
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+// WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+// License for the specific language governing permissions and limitations
+// under the License.
+// ----------------------------------------------------------------------------
+
+#include "astcenc_mathlib.h"
+
+/**
+ * @brief 64-bit rotate left.
+ *
+ * @param val   The value to rotate.
+ * @param count The rotation, in bits.
+ */
+static inline uint64_t rotl(uint64_t val, int count)
+{
+	return (val << count) | (val >> (64 - count));
+}
+
+/* See header for documentation. */
+void astc::rand_init(uint64_t state[2])
+{
+	state[0] = 0xfaf9e171cea1ec6bULL;
+	state[1] = 0xf1b318cc06af5d71ULL;
+}
+
+/* See header for documentation. */
+uint64_t astc::rand(uint64_t state[2])
+{
+	uint64_t s0 = state[0];
+	uint64_t s1 = state[1];
+	uint64_t res = s0 + s1;
+	s1 ^= s0;
+	state[0] = rotl(s0, 24) ^ s1 ^ (s1 << 16);
+	state[1] = rotl(s1, 37);
+	return res;
+}