refactor(physics): promote ACDREAM_DUMP_STEEP_ROOF into PhysicsDiagnostics

First application of CLAUDE.md's new Code Structure Rules §5
("Runtime probes belong in diagnostic owner classes"). Migrates the
four ACDREAM_DUMP_STEEP_ROOF call-site env reads into a single
PhysicsDiagnostics.DumpSteepRoofEnabled property initialized from the
env var at type init, with a runtime setter that follows the existing
ProbeResolveEnabled / ProbeCellEnabled / ProbeBuildingEnabled pattern.

Sites migrated:
  - AcDream.Core/Physics/PhysicsEngine.cs:637 (KILL-VELOCITY-APPLIED log)
  - AcDream.Core/Physics/TransitionTypes.cs:718 (PHASE3-RESET log)
  - AcDream.App/Input/PlayerMovementController.cs:1117 (FRAME log)
  - AcDream.App/Input/PlayerMovementController.cs:1199 (per-frame bounce log)

Behavior-preservation only. ACDREAM_DUMP_STEEP_ROOF=1 still produces
identical [steep-roof] log output. The class-comment in
PhysicsDiagnostics already anticipated this migration
("Future slices may fold the older ACDREAM_DUMP_* env vars into this
class for unified runtime toggling").

Not yet wired to a DebugVM checkbox — runtime toggling is available
via the property setter for future debugging sessions, but exposing
it on the panel is a 30-second future cut, not in scope here.

Build: green.
Tests: same pass/fail profile as before this commit (8 pre-existing
Core failures unrelated to physics-diagnostics; App.Tests / Core.Net.Tests
/ UI.Abstractions.Tests all green).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/AcDream.App/Input/PlayerMovementController.cs

@@ -1114,7 +1114,7 @@ public sealed class PlayerMovementController
         // L.4-diag (2026-04-30): trace position transitions so we can see
         // whether the body is actually moving frame-to-frame on the steep
         // roof, or whether it's frozen at the impact point.
-        if (Environment.GetEnvironmentVariable("ACDREAM_DUMP_STEEP_ROOF") == "1"
+        if (AcDream.Core.Physics.PhysicsDiagnostics.DumpSteepRoofEnabled
             && resolveResult.CollisionNormalValid)
         {
             Console.WriteLine(
@@ -1196,7 +1196,7 @@ public sealed class PlayerMovementController
                 : (!prevOnWalkable && !nowOnWalkable);
 
             // L.4-diag (2026-04-30): per-frame bounce trace for steep-roof bug.
-            bool diagSteep = Environment.GetEnvironmentVariable("ACDREAM_DUMP_STEEP_ROOF") == "1";
+            bool diagSteep = AcDream.Core.Physics.PhysicsDiagnostics.DumpSteepRoofEnabled;
             if (diagSteep && resolveResult.CollisionNormalValid)
             {
                 var n0 = resolveResult.CollisionNormal;

src/AcDream.Core/Physics/PhysicsDiagnostics.cs

@@ -141,4 +141,29 @@ public static class PhysicsDiagnostics
     /// </summary>
     public static bool ProbeUseabilityFallbackEnabled { get; set; } =
         Environment.GetEnvironmentVariable("ACDREAM_PROBE_USEABILITY_FALLBACK") == "1";
+
+    /// <summary>
+    /// L.4-diag (2026-04-30) → promoted into <see cref="PhysicsDiagnostics"/>
+    /// 2026-05-16 per CLAUDE.md "Code Structure Rules" §5 (diagnostic owner
+    /// classes, not per-call-site env reads). Gates the <c>[steep-roof]</c>
+    /// trace family that fires from four sites during the rooftop-bounce
+    /// investigation:
+    /// <list type="bullet">
+    ///   <item><description><c>PhysicsEngine.ResolveWithTransition</c> —
+    ///     <c>[steep-roof] KILL-VELOCITY-APPLIED</c> when retail-faithful
+    ///     <c>kill_velocity</c> zeroes the body's velocity on steep-slope
+    ///     impact.</description></item>
+    ///   <item><description><c>TransitionTypes</c> (<c>FindEnvCollisions</c>
+    ///     post-step) — per-frame plane-normal trace on the active
+    ///     <see cref="CollisionInfo"/>.</description></item>
+    ///   <item><description><c>PlayerMovementController</c> — two sites
+    ///     emitting <c>[steep-roof]</c> + the per-frame bounce trace when
+    ///     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
+    /// follow-up if a debugging session calls for it).
+    /// </summary>
+    public static bool DumpSteepRoofEnabled { get; set; } =
+        Environment.GetEnvironmentVariable("ACDREAM_DUMP_STEEP_ROOF") == "1";
 }

src/AcDream.Core/Physics/PhysicsEngine.cs

@@ -634,7 +634,7 @@ public sealed class PhysicsEngine
             //   acclient_2013_pseudo_c.txt:272567 (validate_transition)
             if (transition.ObjectInfo.VelocityKilled)
             {
-                if (Environment.GetEnvironmentVariable("ACDREAM_DUMP_STEEP_ROOF") == "1")
+                if (PhysicsDiagnostics.DumpSteepRoofEnabled)
                     Console.WriteLine($"[steep-roof] KILL-VELOCITY-APPLIED Vbefore=({body.Velocity.X:F2},{body.Velocity.Y:F2},{body.Velocity.Z:F2}) → 0,0,0");
                 body.Velocity = Vector3.Zero;
             }

src/AcDream.Core/Physics/TransitionTypes.cs

@@ -715,7 +715,7 @@ public sealed class Transition
                     ci.ContactPlaneValid   = false;
                     ci.ContactPlaneIsWater = false;
 
-                    bool diagSteep = Environment.GetEnvironmentVariable("ACDREAM_DUMP_STEEP_ROOF") == "1";
+                    bool diagSteep = PhysicsDiagnostics.DumpSteepRoofEnabled;
                     if (diagSteep)
                     {
                         Console.WriteLine(