fix #434: delete the unreachable DebugPanel/DebugVM surface and the comments that advertised it

DebugPanel and DebugVM have not been constructible since Campaign V slice
V11 removed the ImGui frontend that hosted them: nothing in src/ ever
called their constructors, only a test did. Two consequences, both fixed
here — 35 environment reads inside them were unreachable, and roughly forty
XML doc comments across the diagnostics owners promised a runtime checkbox
that no longer exists. A flag documented as runtime-toggleable when it is
startup-only sends the next investigation down a path that cannot work.

Deleted DebugPanel.cs (340 lines), DebugVM.cs (548) and DebugVMTests.cs
(327). Corrected the surviving claims in PhysicsDiagnostics,
RenderingDiagnostics, CameraDiagnostics, PhysicsEngine and GameWindow to say
what is actually true: these flags are set from the environment at startup
or by direct assignment.

The one real dependant was CombatFeedbackSlot, whose binding target was
DebugVM. It now takes a plain Action<string>, which removes the dependency
without changing behavior — and makes visible that there is no behavior:
nothing binds the slot, so the combat refusals it carries ("No monster
target", "Enter melee or missile combat first") have been discarded all
along. Filed as #436 and pinned by a test, rather than papered over with an
invented chat message; the retail text and channel need the oracle first.

Deliberately untouched: F1's AcdreamToggleDebugPanel binding, which
GameplayInputCommandController consumes as a documented no-op so the key
does not fall through to a lower input scope; and the
DebugVmRenderFactsPublisher / DevToolsRuntimeSources chain, which is still
wired into production composition and deserves its own dead-code pass
instead of being pulled into this one.

Full hermetic suite 15,333 passed / 0 failed.

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

tests/AcDream.App.Tests/Combat/CombatFeedbackSlotTests.cs

@@ -1,7 +1,4 @@
-using System.Numerics;
 using AcDream.App.Combat;
-using AcDream.Core.Combat;
-using AcDream.UI.Abstractions.Panels.Debug;
 
 namespace AcDream.App.Tests.Combat;
 
@@ -11,49 +8,41 @@ public sealed class CombatFeedbackSlotTests
     public void ExpectedOwnerUnbindCannotClearReplacement()
     {
         var slot = new CombatFeedbackSlot();
-        using DebugVM first = CreateViewModel();
-        using DebugVM second = CreateViewModel();
+        List<string> first = [];
+        List<string> second = [];
+        Action<string> firstTarget = first.Add;
+        Action<string> secondTarget = second.Add;
 
-        slot.Bind(first);
-        slot.Unbind(second);
+        slot.Bind(firstTarget);
+        slot.Unbind(secondTarget);
         slot.Show("first");
-        Assert.Single(first.RecentToasts);
-        Assert.Empty(second.RecentToasts);
+        Assert.Equal(["first"], first);
+        Assert.Empty(second);
 
-        slot.Unbind(first);
-        slot.Bind(second);
+        slot.Unbind(firstTarget);
+        slot.Bind(secondTarget);
         slot.Show("second");
-        Assert.Single(second.RecentToasts);
+        Assert.Equal(["second"], second);
     }
 
-    private static DebugVM CreateViewModel() => new(
-        static () => Vector3.Zero,
-        static () => 0,
-        static () => 0,
-        static () => false,
-        static () => false,
-        static () => false,
-        static () => 0,
-        static () => 0,
-        static () => 0,
-        static () => 0,
-        static () => 0,
-        static () => 0,
-        static () => float.PositiveInfinity,
-        static () => "-",
-        static () => false,
-        static () => false,
-        static () => 0,
-        static () => 1,
-        static () => 0,
-        static () => false,
-        static () => "0",
-        static () => 0,
-        static () => "Clear",
-        static () => 0,
-        static () => 0,
-        static () => 0,
-        static () => 60,
-        static () => 16.7f,
-        new CombatState());
+    [Fact]
+    public void RebindingADifferentTargetWhileBoundIsRejected()
+    {
+        var slot = new CombatFeedbackSlot();
+        slot.Bind(_ => { });
+
+        Assert.Throws<InvalidOperationException>(() => slot.Bind(_ => { }));
+    }
+
+    [Fact]
+    public void AnUnboundSlotDropsItsMessages()
+    {
+        // #434/#436: this is the shipped behavior, not an aspiration —
+        // nothing binds the slot in production, so combat refusal text
+        // ("No monster target") is discarded. Pinned so the day it gets a
+        // real binder, this test is the one that has to change.
+        var slot = new CombatFeedbackSlot();
+
+        slot.Show("dropped");
+    }
 }