test(storage): exercise AppPaths' full data-root precedence chain

DefaultDataRoot, the pointer-file location and the portable-mode root
all read machine-global locations (APPDATA, AppContext.BaseDirectory),
so the override→portable→pointer→default precedence in DataRoot/
IsDataLocationConfigured was previously untestable beyond the override
case. Added three internal overrides (BootstrapDirOverride,
PortableBaseOverride, DefaultRootOverride) as sandboxing seams, and a
new AppPathsResolutionTests class covering every precedence branch:
default-only, portable-beats-pointer, pointer-beats-default, a blank
pointer falling back to default, "already has data" counting as
configured, and override beating everything. Both AppPaths test
classes share the "AppPaths" xUnit collection since this state is
static.

src/Core/Storage/AppPaths.cs

@@ -21,6 +21,13 @@ public static class AppPaths
     /// <summary>Explicit override of the data root (e.g. CLI --data-dir). Takes priority over everything.</summary>
     public static string? OverrideDataRoot { get; set; }
 
+    // Test seams: redirect the machine-global bootstrap locations (APPDATA pointer
+    // dir, executable dir, per-user default root) to a sandbox so the resolution
+    // precedence can be exercised without touching real user data.
+    internal static string? BootstrapDirOverride { get; set; }
+    internal static string? PortableBaseOverride { get; set; }
+    internal static string? DefaultRootOverride { get; set; }
+
     /// <summary>
     /// Default data root, following each platform's convention:
     /// Windows → %APPDATA%\PalladiumWallet (PascalCase, like Electrum/Bitcoin);
@@ -29,6 +36,8 @@ public static class AppPaths
     /// </summary>
     public static string DefaultDataRoot()
     {
+        if (DefaultRootOverride is { } o)
+            return o;
         if (OperatingSystem.IsWindows())
             return Path.Combine(
                 Environment.GetFolderPath(Environment.SpecialFolder.ApplicationData),
@@ -41,11 +50,12 @@ public static class AppPaths
     /// bootstrap location that is always writable and independent of the data root.</summary>
     private static string LocationPointerPath() =>
         Path.Combine(
-            Environment.GetFolderPath(Environment.SpecialFolder.ApplicationData),
-            AppDirName, "data-location");
+            BootstrapDirOverride ?? Path.Combine(
+                Environment.GetFolderPath(Environment.SpecialFolder.ApplicationData), AppDirName),
+            "data-location");
 
     private static string PortableRoot() =>
-        Path.Combine(AppContext.BaseDirectory, PortableDirName);
+        Path.Combine(PortableBaseOverride ?? AppContext.BaseDirectory, PortableDirName);
 
     private static bool HasData(string root) =>
         Directory.Exists(root) && Directory.EnumerateFileSystemEntries(root).Any();