Fix doubled platformdirs paths; pick newest dotnet runtime

appauthor=False/opinion=False avoids mediahive\mediahive\Cache style
paths on Windows. fetch_dotnet now probes all known dotnet locations and
selects the highest Microsoft.NETCore.App major, requiring the version
vpk's TFM targets (10) — PATH on the Windows CI runner resolves to a
runtime-only .NET 8 while scoop holds the SDK 10.

scripts/guibuild.py

@@ -47,7 +47,7 @@ _ASSETS_DIR = _REPO_ROOT / "mediahive" / "assets"
 
 def _build_cache_dir() -> Path:
     """Return the persistent cross-build cache dir for downloaded tools (CI wipes build/)."""
-    return user_cache_path("mediahive-build")
+    return user_cache_path("mediahive-build", appauthor=False, opinion=False)
 
 
 _FFMPEG_STAGING = _build_cache_dir() / "ffmpeg"
@@ -202,12 +202,16 @@ def ensure_macos_icon() -> Path:
     return icon_icns
 
 
+_VPK_TFM = "net10.0"
+_VPK_REQUIRED_DOTNET_MAJOR = int(re.fullmatch(r"net(\d+)\.0", _VPK_TFM).group(1))
+
+
 def fetch_vpk() -> Path:
     """Download the Velopack CLI package into the persistent build cache.
 
     Returns the path to vpk.dll, runnable with `dotnet vpk.dll ...`.
     """
-    vpk_dll = _VPK_STAGING / "tools" / "net10.0" / "any" / "vpk.dll"
+    vpk_dll = _VPK_STAGING / "tools" / _VPK_TFM / "any" / "vpk.dll"
     if vpk_dll.exists():
         print(f"vpk already staged at {_VPK_STAGING}, skipping download.")
         return vpk_dll
@@ -226,35 +230,35 @@ def fetch_vpk() -> Path:
     return vpk_dll
 
 
-def _dotnet_has_runtime(exe: Path) -> bool:
-    """Check that `exe` runs and has Microsoft.NETCore.App >= 8."""
+def _dotnet_runtime_major(exe: Path) -> int | None:
+    """Return the highest installed Microsoft.NETCore.App major version, or None."""
     try:
         result = subprocess.run(
             [str(exe), "--list-runtimes"], capture_output=True, text=True, timeout=30
         )
     except (OSError, subprocess.TimeoutExpired):
-        return False
+        return None
     if result.returncode != 0:
-        return False
+        return None
+    majors = []
     for line in result.stdout.splitlines():
         parts = line.split()
         if len(parts) >= 2 and parts[0] == "Microsoft.NETCore.App":
             try:
-                if int(parts[1].split(".")[0]) >= 8:
-                    return True
+                majors.append(int(parts[1].split(".")[0]))
             except ValueError:
                 continue
-    return False
+    return max(majors, default=None)
 
 
 def fetch_dotnet() -> str:
-    """Resolve a dotnet host with a modern (>= 8) runtime from the system.
+    """Resolve a system dotnet host able to run vpk (needs .NET >= 10).
 
     The .NET SDK is a build prerequisite installed on the build machine —
-    downloading a runtime per build is slow and flaky. A plain `dotnet` may
-    resolve to a runtime-only installation, and long-running services (CI
-    runners) may carry a stale environment without DOTNET_ROOT or scoop
-    paths — so probe known locations explicitly.
+    downloading a runtime per build is slow and flaky. Several dotnet
+    installations may coexist (PATH may resolve to a runtime-only .NET 8
+    while scoop holds the SDK 10), so probe known locations and pick the
+    newest runtime rather than the first that runs.
     """
     exe_name = "dotnet.exe" if sys.platform == "win32" else "dotnet"
     candidates: list[Path] = []
@@ -281,14 +285,22 @@ def fetch_dotnet() -> str:
             Path.home() / ".dotnet" / exe_name,
         ]
 
+    best: tuple[int, Path] | None = None
     for exe in candidates:
-        if exe.exists() and _dotnet_has_runtime(exe):
-            print(f"Using dotnet at {exe}")
-            return str(exe)
+        if not exe.exists():
+            continue
+        major = _dotnet_runtime_major(exe)
+        if major is not None and (best is None or major > best[0]):
+            best = (major, exe)
 
+    if best is not None and best[0] >= _VPK_REQUIRED_DOTNET_MAJOR:
+        print(f"Using dotnet at {best[1]} (.NET {best[0]})")
+        return str(best[1])
+
+    found = f"newest found is .NET {best[0]} at {best[1]}" if best else "none found"
     raise RuntimeError(
-        "No dotnet with Microsoft.NETCore.App >= 8 found. "
-        "Install the .NET SDK on this build machine "
+        f"vpk requires Microsoft.NETCore.App >= {_VPK_REQUIRED_DOTNET_MAJOR} ({found}). "
+        "Install the current .NET SDK on this build machine "
         "(Windows: `scoop install dotnet-sdk`; macOS: `brew install dotnet-sdk`; "
         "Linux: distro `dotnet-sdk` package or the dotnet-install script)."
     )