Require system .NET SDK; cache vpk/ffmpeg persistently

Per-build downloads of the dotnet runtime (~80 MB) were slow and flaky
(IncompleteRead killed a CI run). The .NET SDK is now a build-host
prerequisite; vpk and ffmpeg download once into a user-level cache
(~/.cache/mediahive-build, %LOCALAPPDATA%\mediahive-build) that
survives the per-run build dir.

scripts/MediaHive.spec

@@ -5,8 +5,9 @@
 #     pyinstaller --noconfirm --clean scripts/MediaHive.spec
 #
 # Or use the build script (recommended—handles versioning and packaging):
-#   uv run scripts/winbuild.py
+#   uv run scripts/guibuild.py
 
+import os
 import sys
 import mediahive.winmain
 import mediahive.server
@@ -20,7 +21,15 @@ _frontend_build = _pkg / "frontend-build"
 _logo_webp = _pkg / "assets" / "mediahive.webp"
 _icon_win = _pkg / "assets" / "mediahive.ico"
 _icon_mac = _pkg / "assets" / "mediahive.icns"
-_tools_dir = Path(SPECPATH).parent / "build" / "ffmpeg"
+# ffmpeg staging lives in the persistent build cache (same logic as
+# scripts/guibuild.py); fall back to the legacy build/ffmpeg location.
+if sys.platform == "win32":
+    _cache_base = Path(os.environ.get("LOCALAPPDATA", Path.home() / "AppData" / "Local"))
+else:
+    _cache_base = Path(os.environ.get("XDG_CACHE_HOME", Path.home() / ".cache"))
+_tools_dir = _cache_base / "mediahive-build" / "ffmpeg"
+if not _tools_dir.exists():
+    _tools_dir = Path(SPECPATH).parent / "build" / "ffmpeg"
 _tool_names = ["ffmpeg.exe"] if sys.platform == "win32" else ["ffmpeg"]
 
 _binaries = []