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>

src/AcDream.App/Rendering/GameWindow.cs

@@ -134,8 +134,8 @@ public sealed class GameWindow :
     private DebugLineRenderer? _debugLines;
     // K-fix4 (2026-04-26): default OFF. The orange BSP / green cylinder
     // wireframes are noisy outdoors and confuse first-time users into
-    // thinking they're a rendering bug. Ctrl+F2 toggles, the DebugPanel
-    // → Diagnostics → "Toggle collision wires" button toggles too.
+    // thinking they're a rendering bug. Ctrl+F2 toggles. (The DebugPanel
+    // button that also toggled it is gone — #434.)
     private readonly AcDream.App.Rendering.WorldSceneDebugState
         _worldSceneDebugState = new();