feat(plugins): enforce apiVersion; launcher plugins default ON with "none" opt-out

Two gaps from the MossTank shipment review.

**apiVersion was declared in every manifest and checked by nothing.** The
loader now refuses an unsupported contract BEFORE loading any code from the
plugin — checking after the fact is not equivalent, because by then the
assembly is in a collectible context and the mismatch surfaces as a type-load
or missing-member failure from inside the plugin, which reads like the plugin
is broken rather than built for a different host. PluginApi (Current /
MinimumSupported) lives in Plugin.Abstractions beside the contract it
versions, and the refusal is a distinct PluginApiVersionException so callers
can tell "update the client or the plugin" from "this plugin is broken". The
tests pin the ordering too: a manifest with a future apiVersion AND a missing
dll must fail on the version, a supported one on the dll.

**A launcher-launched client loaded no plugins until the user typed ids.**
LA5 distinguishes an omitted allow-list (load all) from an explicit empty one
(load none); a fresh character profile's list is empty, so it composed to
load-none. Direct launches pass null and load everything -- which is why the
gap never showed in development: the two launch paths disagreed and the
launcher was the one users get. This REVERSES the LA5 default deliberately:
"nothing configured" now composes to the omitted list, so plugins are on by
default, including ones installed later. The opt-out is kept -- losing it
would be a real regression for stripped sessions -- respelled as the literal
id "none", and the launcher's plugin box says so.

The cross-host shared fixture composes its explicit-load-none case through
the new spelling, keeping the reader-side contract tests (App and Headless
both preserve an explicit empty list) exactly as they were.

Complete Release suite: 14,469 tests pass on the standard hermetic lane
filter, 0 failures.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/AcDream.Core/Plugins/PluginLoader.cs

@@ -20,6 +20,21 @@ public static class PluginLoader
         ArgumentNullException.ThrowIfNull(manifest);
         ArgumentNullException.ThrowIfNull(host);
 
+        // Refuse a contract we cannot honour BEFORE loading any code from it.
+        // Checking after the fact is not equivalent: the assembly is already in
+        // a collectible context, and the mismatch surfaces as a type-load or
+        // missing-member failure from inside the plugin, which reads like the
+        // plugin is broken rather than built for a different host.
+        if (!PluginApi.IsSupported(manifest.ApiVersion))
+            return new LoadedPlugin(
+                manifest,
+                Plugin: null,
+                LoadContext: null,
+                Error: new PluginApiVersionException(
+                    $"plugin '{manifest.Id}' declares apiVersion {manifest.ApiVersion}, "
+                    + $"but this build supports {PluginApi.MinimumSupported}"
+                    + $"..{PluginApi.Current}"));
+
         var dllPath = Path.Combine(pluginDirectory, manifest.EntryDll);
         if (!File.Exists(dllPath))
             return new LoadedPlugin(