feat(camera): add CameraDiagnostics static tunable owner

Six knobs for the upcoming retail chase camera: UseRetailChaseCamera
master toggle (env ACDREAM_RETAIL_CHASE), AlignToSlope (env
ACDREAM_CAMERA_ALIGN_SLOPE, default on), TranslationStiffness +
RotationStiffness (both 0.45 retail default), MouseLowPassWindowSec
(0.25), CameraAdjustmentSpeed (40.0). DebugPanel mirror lands later;
this commit just stands up the static surface + defaults + tests.

Per spec docs/superpowers/specs/2026-05-18-retail-chase-camera-design.md.

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

src/AcDream.Core/Rendering/CameraDiagnostics.cs

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+using System;
+
+namespace AcDream.Core.Rendering;
+
+/// <summary>
+/// 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.
+///
+/// <para>
+/// Spec: <c>docs/superpowers/specs/2026-05-18-retail-chase-camera-design.md</c>.
+/// </para>
+/// </summary>
+public static class CameraDiagnostics
+{
+    /// <summary>
+    /// Master toggle. When false (default) the legacy
+    /// <c>AcDream.App.Rendering.ChaseCamera</c> is the active camera;
+    /// when true, the retail-faithful <c>RetailChaseCamera</c> is.
+    /// Initial state from <c>ACDREAM_RETAIL_CHASE=1</c>.
+    /// </summary>
+    public static bool UseRetailChaseCamera { get; set; } =
+        Environment.GetEnvironmentVariable("ACDREAM_RETAIL_CHASE") == "1";
+
+    /// <summary>
+    /// When true (default), the camera basis follows the player's
+    /// 5-frame averaged velocity vector — tilts with the terrain on
+    /// hills. When false, the basis is built from a flat (yaw, 0) vector
+    /// and the camera stays horizontal even on slopes. Initial state
+    /// from <c>ACDREAM_CAMERA_ALIGN_SLOPE</c>; default-on if unset.
+    /// </summary>
+    public static bool AlignToSlope { get; set; } =
+        Environment.GetEnvironmentVariable("ACDREAM_CAMERA_ALIGN_SLOPE") != "0";
+
+    /// <summary>
+    /// Per-frame translation damping rate. Retail default 0.45. Higher
+    /// (→ 1.0) snaps faster; lower (→ 0.0) lags more. Formula per frame:
+    /// <c>alpha = clamp(TranslationStiffness * dt * 10, 0, 1)</c>.
+    /// </summary>
+    public static float TranslationStiffness { get; set; } = 0.45f;
+
+    /// <summary>
+    /// Per-frame rotation damping rate. Independent of translation —
+    /// can be tuned higher so the camera swings to look at you faster
+    /// than it physically catches up. Retail default 0.45.
+    /// </summary>
+    public static float RotationStiffness { get; set; } = 0.45f;
+
+    /// <summary>
+    /// Mouse-delta low-pass window (seconds). Mouse deltas spaced
+    /// closer than this are averaged with the previous delta before
+    /// being fed to pitch/yaw adjustments. Smooths out jitter on
+    /// high-DPI mice. Retail default 0.25.
+    /// </summary>
+    public static float MouseLowPassWindowSec { get; set; } = 0.25f;
+
+    /// <summary>
+    /// Per-second rate that held-key offset adjustments
+    /// (CameraZoomIn/Out, CameraRaise/Lower) integrate into the
+    /// camera's Distance / Pitch. Retail default 40.0.
+    /// </summary>
+    public static float CameraAdjustmentSpeed { get; set; } = 40.0f;
+}

tests/AcDream.Core.Tests/Rendering/CameraDiagnosticsTests.cs

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+using AcDream.Core.Rendering;
+using Xunit;
+
+namespace AcDream.Core.Tests.Rendering;
+
+public class CameraDiagnosticsTests
+{
+    // NOTE: These tests assume the env vars ACDREAM_RETAIL_CHASE and
+    // ACDREAM_CAMERA_ALIGN_SLOPE are NOT set when running the test
+    // suite. Static class is initialised on first access; values reflect
+    // the env state at that time.
+
+    [Fact]
+    public void Defaults_AreRetailValues()
+    {
+        // Reset to defaults explicitly (test isolation; another test may
+        // have flipped these earlier in the run).
+        CameraDiagnostics.TranslationStiffness  = 0.45f;
+        CameraDiagnostics.RotationStiffness     = 0.45f;
+        CameraDiagnostics.MouseLowPassWindowSec = 0.25f;
+        CameraDiagnostics.CameraAdjustmentSpeed = 40.0f;
+        CameraDiagnostics.AlignToSlope          = true;
+        CameraDiagnostics.UseRetailChaseCamera  = false;
+
+        Assert.Equal(0.45f, CameraDiagnostics.TranslationStiffness);
+        Assert.Equal(0.45f, CameraDiagnostics.RotationStiffness);
+        Assert.Equal(0.25f, CameraDiagnostics.MouseLowPassWindowSec);
+        Assert.Equal(40.0f, CameraDiagnostics.CameraAdjustmentSpeed);
+        Assert.True(CameraDiagnostics.AlignToSlope);
+        Assert.False(CameraDiagnostics.UseRetailChaseCamera);
+    }
+
+    [Fact]
+    public void Setters_PersistRuntimeChanges()
+    {
+        CameraDiagnostics.TranslationStiffness = 0.8f;
+        CameraDiagnostics.UseRetailChaseCamera = true;
+
+        Assert.Equal(0.8f, CameraDiagnostics.TranslationStiffness);
+        Assert.True(CameraDiagnostics.UseRetailChaseCamera);
+
+        // Reset so other tests aren't poisoned.
+        CameraDiagnostics.TranslationStiffness = 0.45f;
+        CameraDiagnostics.UseRetailChaseCamera = false;
+    }
+}