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>

docs/launch-options.md

@@ -238,6 +238,7 @@ $env:ACDREAM_FRAME_HISTORY            = "$scratch\frames.csv"
 | `ACDREAM_VULKAN_FORCE_UNSUPPORTED` | `=<feature-name>` (case-insensitive property name, e.g. `MultiDrawIndirect`) | Test knob (Slice V5): clears one named required Vulkan feature from the capability record to synthetically fail the gate, so the `NotSupportedException` → exit-code-4 → report path can be exercised on hardware that actually supports everything. | Deliberately breaks Vulkan startup when set to a matched feature name — this is a "make it fail on purpose" gate-testing flag, never appropriate for a normal or measurement run. | `null` → real capabilities used unmodified | `RuntimeOptions.VulkanForcedUnsupportedFeature` → `VulkanCapabilityRecord.Without` (`VulkanCapabilityRecord.cs:113-119`), consumed at `VulkanGraphicsContext.cs:339` |
 | `ACDREAM_VULKAN_PROBE` | `=1` | Runs the standalone Vulkan capability-probe/bring-up harness (opens its own window, runs the capability gate, presents synthetic V6c/V6d verification scenes, captures one screenshot) **instead of** the real client composition host, then exits. | This flag ALONE gates entry (`GameWindow.cs:828`); the former `ACDREAM_RENDER_BACKEND=vulkan` co-requisite died with the OpenGL backend (its class doc was corrected 2026-08-24). | `false` → normal composition host | `RuntimeOptions.VulkanCapabilityProbe` → `GameWindow.cs:828` → `VulkanBringUpHost` |
 | `ACDREAM_VULKAN_PROBE_FRAMES` | `=<int>` (non-negative) | Bounds the bring-up probe harness to N presented frames so it can run unattended in CI, instead of presenting until a human closes the window. | The frame budget never cuts a pending screenshot capture short — the loop stays open until the screenshot has been attempted even past the budget, so an unattended run's whole product (a PNG) is guaranteed. Zero (unset/unparseable/explicit `0`) keeps the interactive wait-for-close behavior. | `0` → interactive (wait for window close) | `RuntimeOptions.VulkanCapabilityProbeFrames` → `VulkanBringUpHost.cs:141-249` |
+| `ACDREAM_VTANK_PROFILE_DIR` | `=<path>` | Overrides the directory `IPluginHost.VtankProfiles` (a VTank-compatible plugin's real `.usd`/`.ast`/`.af` profile storage — see `AcDream.Plugins.MossTank.VtankProfileDirectory`) is rooted at, composed as a `FilePluginStorage`. Set it to a real installed VTank's own profile folder (e.g. `C:\Games\VirindiPlugins\VirindiTank`) for direct interop. | Redirects only that one plugin-storage root; no other startup behavior changes. An unset/empty-string value is treated as "no override" (`NullIfEmpty`); whitespace-only is NOT special-cased (matches every other `NullIfEmpty`-read flag, e.g. `ACDREAM_AC_DIR`/`ACDREAM_UI_PROBE_SCRIPT`) and would be used as a literal (almost certainly invalid) root. | unset → `<ApplicationPathSet.DataDirectory>/vtank` | `RuntimeOptions.VtankProfileDirectoryOverride` → `Program.cs` (composes `AppPluginHost`'s `vtankProfiles` argument). The Headless host has no equivalent override (its path overrides are `HeadlessPathOverrides`, not env vars) and always uses `HeadlessPathSet.VtankProfilesDirectory` (`<DataDirectory>/vtank`). |
 | `ACDREAM_DUMP_MOVE_TRUTH` | `=1` | Emits one `move-truth OUT` line per outbound movement record (MoveToState / AutonomousPosition): local resolved position vs the wire position/cell, ground contact, velocity (`MovementTruthDiagnosticController`). | **Automation apparatus, NOT a spent probe** — the canonical nine-stop soak (`tools/run-connected-r6-soak.ps1`) hard-gates on ≥2 of these lines per destination as its proof that production input produced outbound movement traffic; deleting it fails the soak at every stop (#437, deleted-and-restored 2026-08-24). Print volume follows the outbound send cadence. | off | `RuntimeOptions.DumpMoveTruth` → `GameWindow.cs` → `MovementTruthDiagnosticController` |
 
 ## Permanent diagnostics