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.Core/Rendering/CameraDiagnostics.cs

@@ -6,7 +6,8 @@ namespace AcDream.Core.Rendering;
 /// Runtime-tunable knobs for the retail-faithful chase camera. Mirrors
 /// the <see cref="AcDream.Core.Physics.PhysicsDiagnostics"/> pattern:
 /// static fields seeded from env vars at process start, runtime-settable
-/// via property setters that the DebugPanel writes to.
+/// via property setters. (#434: the DebugPanel that used to write them has
+/// been unreachable since Campaign V slice V11.)
 ///
 /// <para>
 /// Spec: <c>docs/superpowers/specs/2026-05-18-retail-chase-camera-design.md</c>.
@@ -21,7 +22,7 @@ public static class CameraDiagnostics
     /// <c>AcDream.App.Rendering.ChaseCamera</c> rigid-follow camera is.
     /// Initial state from <c>ACDREAM_RETAIL_CHASE</c> — default-on if
     /// unset, off only when explicitly set to <c>"0"</c>. The legacy
-    /// camera stays available via the DebugPanel toggle pending the
+    /// camera stays available by assigning this property pending the
     /// follow-up deletion commit.
     /// </summary>
     public static bool UseRetailChaseCamera { get; set; } =

src/AcDream.Core/Rendering/RenderingDiagnostics.cs

@@ -8,9 +8,11 @@ namespace AcDream.Core.Rendering;
 /// <summary>
 /// 2026-05-19 — runtime-toggleable diagnostic flags for the indoor cell
 /// rendering pipeline. Initialized from env vars at process start;
-/// flippable at runtime via the DebugPanel mirror. Log call sites read
-/// these statics so a checkbox toggle takes effect on the next frame
-/// without relaunching.
+/// flippable at runtime by direct assignment. Log call sites read these
+/// statics so a change takes effect on the next frame without relaunching.
+/// (#434: these used to have a DebugPanel checkbox mirror. That panel has
+/// been unreachable since Campaign V slice V11 removed its ImGui host, so
+/// every flag here is startup-or-assignment only.)
 ///
 /// <para>
 /// Mirrors the L.2a <see cref="AcDream.Core.Physics.PhysicsDiagnostics"/>
@@ -90,8 +92,8 @@ public static class RenderingDiagnostics
     /// Initial state from <c>ACDREAM_PROBE_VIS=1</c>.
     /// <para>
     /// Phase U.2d (2026-05-30) repurposed this flag from the abandoned A8
-    /// two-pipe stencil pass to the Phase U unified pipeline. The env var name +
-    /// the DebugPanel mirror (<c>DebugVM.ProbeVisibility</c>) are unchanged.
+    /// two-pipe stencil pass to the Phase U unified pipeline. The env var name
+    /// is unchanged (its DebugPanel mirror is gone — #434).
     /// </para>
     /// </summary>
     public static bool ProbeVisibilityEnabled { get; set; } =
@@ -352,8 +354,8 @@ public static class RenderingDiagnostics
     /// (the intensity-100 portal purples + the viewer fill off; statics stay);
     /// 3 = raw vLit visualization in the fragment shader (texture ignored).
     /// Discriminates lighting-driven stripes (gone at 1/2, visible in the field
-    /// at 3) from texture/per-pixel machinery (survive 1). Settable for a
-    /// future DebugPanel mirror.
+    /// at 3) from texture/per-pixel machinery (survive 1). Settable at
+    /// runtime by direct assignment.
     /// </summary>
     public static int LightDebugMode { get; set; } =
         int.TryParse(
@@ -771,7 +773,7 @@ public static class RenderingDiagnostics
     /// <c>FrameProfiler</c>'s own class doc. Every backend reports GPU time
     /// through <c>FrameProfiler.RecordGpuSample</c>.
     /// Initial state from <c>ACDREAM_FRAME_PROF=1</c>; runtime-toggleable
-    /// via the DebugPanel mirror (<c>DebugVM.FrameProf</c>).
+    /// by direct assignment (its DebugPanel mirror is gone — #434).
     /// Spec: docs/superpowers/specs/2026-07-05-modern-pipeline-design.md §5.
     /// </summary>
     public static bool FrameProfEnabled { get; set; } =