feat(plugins): MossTank — a self-buffing plugin, and the automation surface it needed

First consumer of acdream's plugin automation surface, and the first slice of
the VTank-class plugin milestone
(docs/research/2026-07-29-vtank-plugin-automation-requirements.md).

MossTank shows a panel with a Buff button; clicking it casts every self-buff
the character is missing, skips what is already in force at an equal or higher
tier, and refreshes what is nearly expired.

The host/plugin line is the load-bearing decision here. The host publishes
spell DATA -- family, tier, difficulty, mana, duration -- plus a cast
primitive with a preflight gate. The plugin owns the POLICY. That is the
architectural conclusion the requirements research reached: VTank's engine
lived in plugin-land, built on Decal's primitives, and baking "best buff for
skill X" into the host would start pulling the engine inward one convenience
at a time.

Why the plan is driven off the spellbook rather than off trained skills, which
is the obvious reading of "buff every trained and specialised skill": the
client cannot honestly make that mapping. The link between a spell and the
stat it modifies arrives from the SERVER in the enchantment message and is
absent from the client's own spell table. What the client does know is which
spells the character has learned -- and a character only learns buffs for the
skills they use, so the spellbook reaches the same set without inventing a
mapping the client has no grounds for.

Surface added, all BCL-only so Plugin.Abstractions keeps its zero project
references:

* ICharacterInfo, ISpellCatalog, IMagicCommands, grouped behind one
  IAutomationSurface so IPluginHost grows by one member rather than three.
* IEvents.Tick. Automation is sequences, not single calls -- a buff pass casts
  several spells and must wait between them. Without a host tick a plugin
  would need its own timer thread re-entering the host off its update thread.
* NoOpAutomationSurface for hosts with no live session, so a plugin keeps one
  code path and checks IsAvailable.

Markup gained <button> and <label>; it previously supported only <meter>, with
a comment promising the rest. Buttons bind onclick to an Action property and
FAIL THE PANEL LOAD if it does not resolve -- a silently dead button is worse
than a panel that refuses to load, because the user clicks and there is
nothing to diagnose. Labels bind through a Func so a status line tracks its
binding instead of freezing at build time.

Enchantment reads use EnchantmentsInEffectSnapshot rather than the raw active
set: retail leaves a weaker same-family enchantment in the registry while a
stronger one is in force, and a plugin asking "am I buffed?" means in force.

BuffPlan is a pure function of (known buffs, active enchantments) precisely so
it can be tested without a session; 9 tests cover tier supersede, the
family-0 no-stack bucket that must not be de-duplicated, expiry refresh, and
plan stability across the rebuilds the tick loop performs.

Solution builds clean; 14,421 tests pass on the standard hermetic lane filter,
0 failures.

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

src/AcDream.Headless/Plugins/HeadlessPluginHost.cs

@@ -50,8 +50,34 @@ internal sealed class HeadlessPluginHost
     public IGameState State => this;
     public IEvents Events => this;
     public ISelectionService Selection => _runtime.ActionOwner.Selection;
+    /// <summary>
+    /// No live-session projection is bound on this host yet, so automation is
+    /// inert rather than absent -- a plugin keeps one code path and checks
+    /// IsAvailable.
+    /// </summary>
+    public IAutomationSurface Automation => NoOpAutomationSurface.Instance;
+
     public IUiRegistry Ui => NoOpUiRegistry.Instance;
 
+    /// <summary>
+    /// Accepted but never raised on this host: the headless bot loop drives its
+    /// policies through Runtime's own scheduler, not through a plugin tick, so
+    /// there is no update to forward. A plugin written against the graphical
+    /// host therefore loads and runs here, it simply never ticks. Wiring it is a
+    /// Slice-K scheduler change rather than a plugin-API one.
+    /// </summary>
+    /// <remarks>
+    /// Written with explicit accessors rather than as a field-like event on
+    /// purpose: a field-like event that is never raised is a compiler warning,
+    /// and this project builds warnings as errors. Discarding here states the
+    /// intent instead of suppressing the diagnostic.
+    /// </remarks>
+    public event Action<double> Tick
+    {
+        add { _ = value; }
+        remove { _ = value; }
+    }
+
     /// <summary>Test-only barrier invoked after the first replay item is
     /// captured while Runtime's exact active-membership read lease is still
     /// held.</summary>