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/Physics/PhysicsDiagnostics.cs

@@ -6,10 +6,12 @@ namespace AcDream.Core.Physics;
 
 /// <summary>
 /// L.2a slice 1 (2026-05-12) — runtime-toggleable physics probe flags.
-/// Initialized from env vars at process start; flippable at runtime via
-/// the DebugPanel mirror (or by direct assignment). Log call sites read
-/// these statics so a checkbox toggle takes effect on the next resolve
-/// without relaunching.
+/// Initialized from env vars at process start; flippable at runtime by
+/// direct assignment. Log call sites read these statics so a change takes
+/// effect on the next resolve without relaunching. (#434: these flags 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>
 /// L.2d slice 1 (2026-05-13) adds <see cref="ProbeBuildingEnabled"/> +
@@ -157,7 +159,7 @@ public static class PhysicsDiagnostics
     ///
     /// <para>
     /// Initial state from <c>ACDREAM_PROBE_BUILDING=1</c>. Mirrorable
-    /// via <c>DebugVM.ProbeBuilding</c> when <c>ACDREAM_DEVTOOLS=1</c>.
+    /// by direct assignment (its DebugVM mirror is gone — #434).
     /// </para>
     ///
     /// <para>
@@ -891,7 +893,7 @@ public static class PhysicsDiagnostics
     /// </para>
     ///
     /// <para>Toggle via env var <c>ACDREAM_PROBE_USEABILITY_FALLBACK=1</c>
-    /// or DebugPanel checkbox.</para>
+    /// by direct assignment.</para>
     /// </summary>
     public static bool ProbeUseabilityFallbackEnabled { get; set; } =
         Environment.GetEnvironmentVariable("ACDREAM_PROBE_USEABILITY_FALLBACK") == "1";
@@ -915,7 +917,7 @@ public static class PhysicsDiagnostics
     ///     the post-collision velocity disagrees with retail.</description></item>
     /// </list>
     /// Initial state from <c>ACDREAM_DUMP_STEEP_ROOF=1</c>. Runtime-toggleable
-    /// via the property setter; not yet wired to a DebugPanel checkbox (open
+    /// via the property setter (open
     /// follow-up if a debugging session calls for it).
     /// </summary>
     public static bool DumpSteepRoofEnabled { get; set; } =
@@ -940,7 +942,7 @@ public static class PhysicsDiagnostics
     ///
     /// <para>
     /// Initial state from <c>ACDREAM_PROBE_INDOOR_BSP=1</c>.
-    /// Runtime-toggleable via DebugPanel.
+    /// Runtime-toggleable by direct assignment.
     /// </para>
     ///
     /// <para>
@@ -967,7 +969,7 @@ public static class PhysicsDiagnostics
     ///     zero poly refs, candidate (b)/(d).</description></item>
     /// </list>
     /// This diagnostic fires at most once per EnvCell (cache is no-op after
-    /// first population). It does NOT have a DebugPanel mirror yet — this is
+    /// first population). This is
     /// a one-shot capture tool, not a persistent toggle. Promote to full
     /// infrastructure after the root cause is identified.
     ///
@@ -1019,7 +1021,7 @@ public static class PhysicsDiagnostics
     ///
     /// <para>
     /// Initial state from <c>ACDREAM_PROBE_WALK_MISS=1</c>.
-    /// No DebugPanel mirror — one-shot diagnostic.
+    /// One-shot diagnostic.
     /// </para>
     ///
     /// <para>
@@ -1047,7 +1049,7 @@ public static class PhysicsDiagnostics
     ///
     /// <para>
     /// Initial state from <c>ACDREAM_PROBE_PUSH_BACK=1</c>.
-    /// Runtime-toggleable via DebugVM mirror.
+    /// Runtime-toggleable by direct assignment.
     /// </para>
     ///
     /// <para>
@@ -2034,7 +2036,7 @@ public static class PhysicsDiagnostics
     ///
     /// <para>
     /// Initial state from <c>ACDREAM_PROBE_STEP_WALK=1</c>. One-shot
-    /// diagnostic; no DebugPanel mirror until the root cause is identified.
+    /// diagnostic.
     /// </para>
     /// </summary>
     public static bool ProbeStepWalkEnabled { get; set; } =