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.Plugin.Abstractions/IAcDreamPlugin.cs

@@ -1,6 +1,39 @@
 // src/AcDream.Plugin.Abstractions/IAcDreamPlugin.cs
 namespace AcDream.Plugin.Abstractions;
 
+/// <summary>The plugin contract's version.</summary>
+/// <remarks>
+/// <para>
+/// A plugin declares the version it was built against in its
+/// <c>plugin.json</c> (<c>apiVersion</c>), and the host refuses to load one
+/// it cannot honour. Without that check a plugin built against a different
+/// contract loads anyway and fails later as a MissingMethodException or a
+/// type-load error from inside the plugin's own code — an error that reads
+/// like the plugin is broken rather than mismatched.
+/// </para>
+/// <para>
+/// Bump <see cref="Current"/> only for a BREAKING change to the interfaces in
+/// this assembly (a removed or re-shaped member). Purely additive changes keep
+/// the number, since a plugin built against the older shape still runs.
+/// </para>
+/// </remarks>
+public static class PluginApi
+{
+    /// <summary>The contract version this build implements.</summary>
+    public const int Current = 1;
+
+    /// <summary>
+    /// The oldest contract version this build can still load. Equal to
+    /// <see cref="Current"/> until a breaking change ships with a
+    /// compatibility path.
+    /// </summary>
+    public const int MinimumSupported = 1;
+
+    /// <summary>Whether a plugin declaring <paramref name="apiVersion"/> can load here.</summary>
+    public static bool IsSupported(int apiVersion)
+        => apiVersion >= MinimumSupported && apiVersion <= Current;
+}
+
 public interface IAcDreamPlugin
 {
     /// <summary>