feat(quest): QT6 — plugins can read the contract tracker

The last piece of QT6's own scope: r10-quest-dialogs.md §11.6's contract half.
IGameState.Contracts exposes what the client structurally knows about quests,
which — per that same research — is the tracker and nothing else. The rest of
§11.6 (chat stream, tells, give, use, confirmations) is other features and
stays out of this campaign.

A pull-through source rather than a pushed mirror. Contracts change rarely and
are already owned canonically, so a second copy would only be a thing to keep
in step; reading through means a plugin cannot observe a stale list.

Both hosts implement it. The headless one carries contract id, stage and
progress but no names — a bot has no dat access — because losing the TEXT is
expected while losing the QUEST would leave a bot silently unable to see what
it is on. Same rule covers a contract the installed dat has never heard of: it
still projects, with empty text and a correct status, rather than vanishing.

The interface member is defaulted so a host predating this campaign still
satisfies IGameState.

Two lazy catalog loads exist (the panel's and this one) rather than one shared
instance. That is deliberate: threading a shared ContractCatalog through three
composition records to avoid reading a 322-row immutable table at most twice
per session would be plumbing for no correctness or performance gain, and the
comment at the call site says so.

Campaign QT is complete; the connected user gate is owed.

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

src/AcDream.Plugin.Abstractions/IGameState.cs

@@ -4,4 +4,15 @@ namespace AcDream.Plugin.Abstractions;
 public interface IGameState
 {
     IReadOnlyList<WorldEntitySnapshot> Entities { get; }
+
+    /// <summary>
+    /// The player's tracked contracts — the client's only structured view of
+    /// quest state (<c>r10-quest-dialogs.md</c> §1.3). Empty when the server
+    /// has sent none.
+    /// </summary>
+    /// <remarks>
+    /// Defaulted so a host predating Campaign QT still satisfies the interface;
+    /// both in-tree hosts implement it.
+    /// </remarks>
+    IReadOnlyList<ContractSnapshot> Contracts => [];
 }