fix(vt): C separate exports/meta/ and exports/nav/ .af directories

Item C (slice-1 fix round). MossTankMetaProfileStore and
MossTankRouteProfileStore both wrote their .af exports into the same
flat "exports/" directory keyed only by profile name — a Meta profile
and a route (Navigation) profile sharing a name (e.g. both named
"Same") would silently clobber each other's .af file on save, with no
error and no warning.

- MossTankMetaProfileStore.WriteLegacyExport now writes to
  "exports/meta/{name}.af".
- MossTankRouteProfileStore.WriteLegacyExport now writes to
  "exports/nav/{name}.af".
- Both class docs updated to name the collision this avoids and point at
  the sibling store's subdirectory.
- MossTankPanelTests: existing NavCommandsImportAndExportExactVtankNavFiles
  and MetaCommandsImportAndExportExactVtankMetFiles updated to the new
  paths. New MetaAndRouteExportsWithTheSameNameDoNotCollide saves a Meta
  and a route profile both named "Same" and asserts both .af files exist
  with their own correct content — verified failing before the fix
  (asserted false on the meta file's existence once both paths were
  reverted to the flat "exports/" root, confirming the collision is real
  and this test catches it).

Full MossTank suite: 569/569 passing (568 -> 569, one new test).

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

src/AcDream.Plugins.MossTank/MossTankMetaProfileStore.cs

@@ -195,7 +195,12 @@ internal sealed class MossTankMetaProfileStore
     /// <c>.af</c> is the only VTank-compatible export format now (Campaign
     /// VT slice 1 Part A — MossTank no longer authors the binary
     /// <c>.met</c> format at all, matching <see cref="VtankMetaProfileSerializer"/>'s
-    /// demotion to a one-shot import).
+    /// demotion to a one-shot import). Exports live under
+    /// <c>exports/meta/</c> — a distinct subdirectory from
+    /// <see cref="MossTankRouteProfileStore"/>'s <c>exports/nav/</c> — so a
+    /// Meta profile and a route (Navigation) profile sharing the same name
+    /// cannot silently overwrite each other's <c>.af</c> file (both stores
+    /// used to write into the same flat <c>exports/</c> root).
     /// </summary>
     private void WriteLegacyExport(string name, MetaProfile profile)
     {
@@ -204,7 +209,7 @@ internal sealed class MossTankMetaProfileStore
         try
         {
             _host.Storage.WriteText(
-                $"exports/{LegacyFileName(name)}.af",
+                $"exports/meta/{LegacyFileName(name)}.af",
                 MetafSerializer.SaveMeta(profile));
         }
         catch (Exception error)

src/AcDream.Plugins.MossTank/MossTankRouteProfileStore.cs

@@ -264,7 +264,12 @@ internal sealed class MossTankRouteProfileStore
     /// <c>.af</c> is the only VTank-compatible export format now (Campaign
     /// VT slice 1 Part A — MossTank no longer authors the binary
     /// <c>.nav</c> format at all, matching <see cref="VtankNavRouteSerializer"/>'s
-    /// demotion to a one-shot import).
+    /// demotion to a one-shot import). Exports live under
+    /// <c>exports/nav/</c> — a distinct subdirectory from
+    /// <see cref="MossTankMetaProfileStore"/>'s <c>exports/meta/</c> — so a
+    /// route (Navigation) profile and a Meta profile sharing the same name
+    /// cannot silently overwrite each other's <c>.af</c> file (both stores
+    /// used to write into the same flat <c>exports/</c> root).
     /// </summary>
     private void WriteLegacyExport(string name, NavigationSettings settings)
     {
@@ -273,7 +278,7 @@ internal sealed class MossTankRouteProfileStore
         try
         {
             _host.Storage.WriteText(
-                $"exports/{LegacyFileName(name)}.af",
+                $"exports/nav/{LegacyFileName(name)}.af",
                 MetafSerializer.SaveNav(settings));
         }
         catch (Exception error)