feat(render): Phase A8.F — RetailChaseCamera consumes the camera-collision probe

Add ICameraCollisionProbe? CollisionProbe { get; init; } to RetailChaseCamera.
Extend Update() with optional cellId/selfEntityId params (default 0) so all
existing callers compile unchanged. After the exponential-damping block (step 5)
and before publishing Position/View (step 6), sweep _dampedEye through the
probe when CameraDiagnostics.CollideCamera is true and a probe is wired in
(step 5b). The fade computation in step 7 then naturally uses the collided eye.
Null probe and cellId=0 both short-circuit cleanly. Three new xUnit tests
cover: probe-wired+flag-on publishes collided eye, flag-off skips probe,
null probe doesn't throw. All 30 RetailChaseCameraTests pass.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/AcDream.App/Rendering/RetailChaseCamera.cs

@@ -52,6 +52,14 @@ public sealed class RetailChaseCamera : ICamera
     /// <summary>Height of look-at anchor above the player's feet (m). Retail default 1.5.</summary>
     public float PivotHeight { get; set; } = 1.5f;
 
+    /// <summary>
+    /// Optional spring-arm collision probe. When set (and
+    /// <see cref="CameraDiagnostics.CollideCamera"/> is true), the damped eye
+    /// is swept from the head-pivot and stopped at the first wall. Null leaves
+    /// the eye uncollided (the default for tests and the legacy path).
+    /// </summary>
+    public ICameraCollisionProbe? CollisionProbe { get; init; }
+
     /// <summary>Computed translucency for the player mesh (0 = opaque, 1 = invisible). Read by GameWindow.</summary>
     public float PlayerTranslucency { get; private set; }
 
@@ -89,7 +97,9 @@ public sealed class RetailChaseCamera : ICamera
         Vector3 playerVelocity,
         bool isOnGround,
         Vector3 contactPlaneNormal,
-        float dt)
+        float dt,
+        uint cellId = 0,
+        uint selfEntityId = 0)
     {
         // 1. Push velocity into 5-frame ring, get average.
         PushVelocity(_velocityRing, ref _velocityCount, playerVelocity);
@@ -132,6 +142,14 @@ public sealed class RetailChaseCamera : ICamera
             _dampedForward = Vector3.Normalize(Vector3.Lerp(_dampedForward, targetForward, rAlpha));
         }
 
+        // 5b. Spring-arm collision (A8.F). Retail SmartBox::update_viewer
+        //     (0x00453ce0) sweeps viewer_sphere from the head-pivot to the
+        //     desired eye and uses the stopped position. Keeps the eye out of
+        //     walls so the A8.F camera-cell + portal side-tests stay stable.
+        //     A null probe or disabled flag leaves the eye unchanged.
+        if (CameraDiagnostics.CollideCamera && CollisionProbe is not null)
+            _dampedEye = CollisionProbe.SweepEye(pivotWorld, _dampedEye, cellId, selfEntityId);
+
         // 6. Publish renderer surface.
         Position = _dampedEye;
         View     = Matrix4x4.CreateLookAt(_dampedEye, _dampedEye + _dampedForward, new Vector3(0f, 0f, 1f));

tests/AcDream.App.Tests/Rendering/RetailChaseCameraTests.cs

@@ -445,4 +445,66 @@ public class RetailChaseCameraTests
         cam.AdjustPitch(+10f);
         Assert.Equal(RetailChaseCamera.PitchMax, cam.Pitch);
     }
+
+    // ── Camera collision (A8.F) ───────────────────────────────────────
+
+    private sealed class FakeProbe : ICameraCollisionProbe
+    {
+        public int Calls;
+        public Vector3 ReturnEye;
+        public Vector3 SweepEye(Vector3 pivot, Vector3 desiredEye, uint cellId, uint selfEntityId)
+        {
+            Calls++;
+            return ReturnEye;
+        }
+    }
+
+    [Fact]
+    public void Update_WithProbeAndFlagOn_PublishesCollidedEye()
+    {
+        CameraDiagnostics.CollideCamera = true;
+        var collided = new Vector3(1f, 2f, 3f);
+        var probe = new FakeProbe { ReturnEye = collided };
+        var cam = new RetailChaseCamera { CollisionProbe = probe };
+
+        cam.Update(
+            playerPosition: Vector3.Zero, playerYaw: 0f, playerVelocity: Vector3.Zero,
+            isOnGround: true, contactPlaneNormal: Vector3.UnitZ, dt: 1f / 60f,
+            cellId: 0x100, selfEntityId: 0x5);
+
+        Assert.True(probe.Calls >= 1);
+        Assert.Equal(collided, cam.Position);
+    }
+
+    [Fact]
+    public void Update_FlagOff_DoesNotConsultProbe()
+    {
+        CameraDiagnostics.CollideCamera = false;
+        var probe = new FakeProbe { ReturnEye = new Vector3(99f, 99f, 99f) };
+        var cam = new RetailChaseCamera { CollisionProbe = probe };
+
+        cam.Update(
+            playerPosition: Vector3.Zero, playerYaw: 0f, playerVelocity: Vector3.Zero,
+            isOnGround: true, contactPlaneNormal: Vector3.UnitZ, dt: 1f / 60f,
+            cellId: 0x100, selfEntityId: 0x5);
+
+        Assert.Equal(0, probe.Calls);
+        Assert.NotEqual(new Vector3(99f, 99f, 99f), cam.Position);
+
+        CameraDiagnostics.CollideCamera = true; // reset
+    }
+
+    [Fact]
+    public void Update_NullProbe_DoesNotThrow()
+    {
+        CameraDiagnostics.CollideCamera = true;
+        var cam = new RetailChaseCamera { CollisionProbe = null };
+
+        cam.Update(
+            playerPosition: Vector3.Zero, playerYaw: 0f, playerVelocity: Vector3.Zero,
+            isOnGround: true, contactPlaneNormal: Vector3.UnitZ, dt: 1f / 60f,
+            cellId: 0x100, selfEntityId: 0x5);
+
+        Assert.NotEqual(default, cam.View);
+    }
 }