fix(vt): F host-composed VtankProfiles storage replaces raw path string

Item F (slice-1 fix round). IPluginHost.VtankProfileDirectory handed the
plugin a raw string path and told it to fall back to its own
System.IO-based portable default when null — a plugin reading and
resolving filesystem paths itself, which is exactly the seam the rest of
IPluginHost.Storage deliberately avoids (Core.Plugins.ScopedPluginHost
scopes/validates every key; the plugin never sees a path).

- IPluginHost: VtankProfileDirectory (string?) deleted; new VtankProfiles
  (IPluginStorage, defaults to NoOpPluginStorage) added — a second,
  UNSCOPED storage instance (unlike Storage, which Core scopes per
  plugin manifest id) rooted at a host-composed VTank-compatible
  directory.
- ScopedPluginHost.VtankProfiles forwards _inner.VtankProfiles directly
  (no scoping — it names one shared external location, not per-plugin
  data). New PluginSessionTests.ScopedHostForwardsVtankProfilesUnscoped
  proves the forwarded instance is the exact same object (Assert.Same),
  not a wrapper.
- AppPluginHost/Program.cs: new vtankProfiles constructor parameter,
  composed as FilePluginStorage(runtimeOptions.VtankProfileDirectoryOverride
  ?? Path.Combine(applicationPaths.DataDirectory, "vtank")).
- RuntimeOptions.VtankProfileDirectoryOverride: new init-only property
  parsed from ACDREAM_VTANK_PROFILE_DIR (row added to
  docs/launch-options.md, side-effects column states the redirect is the
  only effect and documents the NullIfEmpty whitespace-not-special-cased
  quirk it shares with every other path-override flag). New
  RuntimeOptionsTests.VtankProfileDirectoryOverrideIsNullUnlessSet.
- FilePluginStorage.List(prefix): empty prefix now means "the storage
  root itself" instead of throwing (Resolve() rejects empty/whitespace
  keys, which is correct for every OTHER caller but wrong for "list
  everything" — VtankProfileDirectory needs exactly that).
- Headless: HeadlessPluginHost gained the same VtankProfiles
  property/constructor param, threaded through HeadlessPluginSession.Create
  -> HeadlessSessionHost -> HeadlessProcessHost, composed from the new
  HeadlessPathSet.VtankProfilesDirectory (<DataDirectory>/vtank, no
  ACDREAM_VTANK_PROFILE_DIR-equivalent override — Headless path overrides
  are HeadlessPathOverrides/CLI flags, not env vars). A small
  AcDream.Headless.Plugins.FilePluginStorage duplicates the App
  implementation byte-for-byte (Headless does not reference AcDream.App
  and no shared "platform plugins" library exists yet to host one copy;
  documented as a reasonable future consolidation, not required here).
- VtankProfileDirectory.cs rewritten: Resolve/PortableDefault deleted
  outright (no more System.IO, no plugin-owned portable-default fallback);
  ListSettingsProfiles/ListNavigationProfiles/ListMetaProfiles now take
  IPluginStorage and enumerate through EnumerateFileNames, which calls
  storage.List(string.Empty) and skips any key containing '/' (VTank's
  profile directory is flat; a nested key from some other IPluginStorage
  implementation is not a profile file). VtankProfileDirectoryTests
  rewritten against an in-memory IPluginStorage fake instead of real
  temp directories; new NestedPathKeysAreNotTreatedAsProfileFiles pins
  that skip. The prior Resolve/PortableDefault-specific tests (Linux-path
  guarantee, host-override-vs-portable-default) are superseded by
  RuntimeOptionsTests.VtankProfileDirectoryOverrideIsNullUnlessSet plus
  the RuntimeOptions.FromEnvironment Path.Combine-only composition in
  Program.cs.
- docs/architecture/acdream-architecture.md: one sentence in the
  Storage/List(prefix) paragraph naming VtankProfiles as the second,
  unscoped storage.

No production caller of VtankProfileDirectory's listing methods exists
yet (A2's foundation is not wired into MossTankProfileStore/
MossTankMetaProfileStore/MossTankRouteProfileStore's own selection —
per that slice's own ledger note), so this is a contract + plumbing
change with no MossTank runtime behavior change.

MossTank suite: 562/562. Core.Tests (Plugin filter): 50/50. App.Tests
(Plugin|LaunchOptions|RuntimeOptions filter): 135/135. Headless.Tests:
173/174 (the one failure, HeadlessCredentialResolverTests.
LinuxRejectsGroupOrOtherCredentialPermissions, is a pre-existing
Linux-only lane gate that throws PlatformNotSupportedException on this
Windows host — unrelated to this change).

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

src/AcDream.Plugin.Abstractions/IPluginHost.cs

@@ -44,16 +44,20 @@ public interface IPluginHost
     IAutomationSurface Automation { get; }
 
     /// <summary>
-    /// Absolute filesystem directory a VTank-compatible plugin should treat
-    /// as the VTank profile folder (real <c>.usd</c>/<c>.ast</c>/<c>.af</c>
-    /// files, VTank's own naming rules). <see langword="null"/> when the
-    /// host has no opinion, in which case the plugin falls back to its own
-    /// portable per-OS default (never a hard-coded Windows path) — see
-    /// <c>AcDream.Plugins.MossTank.VtankProfileDirectory</c>. A graphical
-    /// host may point this at a real installed VTank's own profile
-    /// directory for direct interop; that platform-specific discovery
-    /// belongs entirely to the host composing this property, never to the
-    /// plugin reading it.
+    /// Storage rooted at the VTank profile folder (real <c>.usd</c>/
+    /// <c>.ast</c>/<c>.af</c> files, VTank's own naming rules) rather than
+    /// this plugin's own scoped <see cref="Storage"/> directory — see
+    /// <c>AcDream.Plugins.MossTank.VtankProfileDirectory</c>, which
+    /// enumerates through this property exclusively (no <c>System.IO</c>,
+    /// no per-OS portable-default fallback of its own) so directory
+    /// discovery stays entirely host-composed. Defaults to the inert
+    /// <see cref="NoOpPluginStorage"/> (<c>IsAvailable</c> false) when the
+    /// host has no opinion. A graphical host may root this at a real
+    /// installed VTank's own profile directory for direct interop, or at
+    /// its own portable per-OS default under
+    /// <c>ApplicationPathSet.DataDirectory</c>; that discovery belongs
+    /// entirely to the host composing this property, never to the plugin
+    /// reading it.
     /// </summary>
-    string? VtankProfileDirectory => null;
+    IPluginStorage VtankProfiles => NoOpPluginStorage.Instance;
 }