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/CollisionShadowVerifierTests.cs

@@ -92,6 +92,36 @@ public sealed class CollisionShadowVerifierTests
         Assert.Equal("False", root.GetProperty("Flat").GetString());
     }
 
+    [Fact]
+    public void FlatAuthority_MismatchKeepsFlatResultAndUsesGraphOnlyAsReferee()
+    {
+        string directory = NewArtifactDirectory();
+        PhysicsDataCache cache = CacheWithShadow(directory);
+        cache.CollisionTraversalMode = CollisionTraversalMode.Flat;
+        CellPhysics matching = MatchingCell(0xA9B4_0100u);
+        CellPhysics mismatched = WithFlatPhysics(
+            matching,
+            new FlatPhysicsBsp(
+                -1,
+                ImmutableArray<FlatPhysicsBspNode>.Empty,
+                ImmutableArray<int>.Empty,
+                FlatPolygonTable.Empty));
+
+        bool flatResult = CollisionTraversal.HasPhysics(cache, mismatched);
+
+        Assert.False(flatResult);
+        CollisionShadowStats stats = cache.CollisionShadowStats;
+        Assert.Equal(1, stats.Queries);
+        Assert.Equal(1, stats.Samples);
+        Assert.Equal(1, stats.Mismatches);
+        Assert.Equal(0, stats.Faults);
+        string artifact = Assert.Single(Directory.EnumerateFiles(directory));
+        using JsonDocument json = JsonDocument.Parse(File.ReadAllText(artifact));
+        JsonElement root = json.RootElement;
+        Assert.Equal("True", root.GetProperty("Graph").GetString());
+        Assert.Equal("False", root.GetProperty("Flat").GetString());
+    }
+
     [Fact]
     public void MissingFlat_IsRecordedAsFaultAndGraphRemainsAuthoritative()
     {