perf(physics): cut production traversal to flat assets

Make prepared flat BSP data authoritative for gameplay while retaining the parsed graph only as an exact sampled referee. Fail production publication when collision package data is genuinely absent, keep idempotent already-cached publication valid, and move cell membership, floor lookup, camera diagnostics, and live/static shape bounds onto the flat representation.

Validated by 8,402 Release tests, a strict dense-Arwic connected gate with 46,309/46,309 exact referee matches, and graceful shutdown.

Co-authored-by: Codex <codex@openai.com>

tests/AcDream.Core.Tests/Physics/PhysicsDataCacheProductionTests.cs

@@ -0,0 +1,61 @@
+using System.Collections.Immutable;
+using AcDream.Core.Physics;
+using DatReaderWriter.DBObjs;
+
+namespace AcDream.Core.Tests.Physics;
+
+public sealed class PhysicsDataCacheProductionTests
+{
+    [Fact]
+    public void CreateProduction_SelectsFlatTraversal()
+    {
+        PhysicsDataCache cache = PhysicsDataCache.CreateProduction();
+
+        Assert.Equal(
+            CollisionTraversalMode.Flat,
+            cache.CollisionTraversalMode);
+    }
+
+    [Fact]
+    public void ProductionSetupPublication_RejectsMissingPreparedAsset()
+    {
+        PhysicsDataCache cache = PhysicsDataCache.CreateProduction();
+
+        InvalidOperationException fault = Assert.Throws<InvalidOperationException>(
+            () => cache.CacheSetup(0x0200_0001u, new Setup()));
+
+        Assert.Contains(
+            "no prepared collision asset",
+            fault.Message,
+            StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void GraphOracleFixture_StillAcceptsSourceSetup()
+    {
+        var cache = new PhysicsDataCache();
+
+        cache.CacheSetup(0x0200_0001u, new Setup());
+
+        Assert.NotNull(cache.GetSetup(0x0200_0001u));
+    }
+
+    [Fact]
+    public void ProductionSetupPublication_AllowsIdempotentCachedFlatAsset()
+    {
+        PhysicsDataCache cache = PhysicsDataCache.CreateProduction();
+        var setup = new Setup();
+        var prepared = new FlatSetupCollision(
+            ImmutableArray<FlatCollisionCylinder>.Empty,
+            ImmutableArray<FlatCollisionSphere>.Empty,
+            0f,
+            0f,
+            0f,
+            0f);
+
+        cache.CacheSetup(0x0200_0001u, setup, prepared);
+        cache.CacheSetup(0x0200_0001u, setup);
+
+        Assert.Same(prepared, cache.GetFlatSetup(0x0200_0001u));
+    }
+}