test: run tile-dependent tests on synthetic terrain instead of client files (#2725)

## Summary

About 114 tests skipped on CI because they needed the official client files (`tiledata.mul`, map and multi files). They gave CI no signal, and locally they ran against whichever client a developer had installed. The test hosts now build their own world instead, so every test runs on CI and runs the same on every machine. CI simulation: Server.Tests 837 passed, UOContent.Tests 1131 passed, **0 skipped**.

- **Generated tile data.** The fixtures write a pre-HS `tiledata.mul` from a small invented tile kit (`Server.Tests/Terrain/TestTiles`) and load it through the real `TileData.Load()`. The test hosts no longer read any client directory (`MODERNUO_CLIENT_PATH`, `MODERNUO_TEST_DATA_DIR` and `C:\Ultima Online Classic` are gone from test code). The file is written to `SyntheticData` in each test project's output, overwritten every run and removed by `dotnet clean`.
- **Injected terrain.** `TestTerrain` stamps land and statics into Felucca and Trammel, which share one terrain. Writes after startup must go in a `TestTerrain.Scope()`, which puts back every block it touched and keeps the step cache fresh.
- **Prefab scenes.** Seven named scenes (town, causeway, sewer walkway, coastline, deep sea, map-edge sea, an upstairs room with an L-shaped counter) replace the Britain coordinates the pathing, AI and fishing tests relied on. Each scene-based test asserts its scene is present first, so a missing scene fails instead of passing on flat ground. Stamping all seven takes 3-5 ms.
- **Multi footprints.** Contrived footprints for the three multi IDs content looks up (0x74, 0x7A, 0x0) are seeded through a new internal `MultiData.SetComponents`.
- **Fingerprint.** `StepCacheFile.HashTileDataFile` is extracted so a test can show that a one-flag change to the file changes the fingerprint, while runtime table mutation does not.

### Engine changes

- `TileMatrix.GetLandBlock`/`GetStaticBlock`: on a map without its files, return the block injected with `SetLandBlock`/`SetStaticBlock`, or the default when nothing was injected. Maps with files take the same path as before.
- `MultiData.SetComponents` (internal): registers a footprint without multi files.

### Test changes worth a look

- Values that were properties of real Britain terrain (probes at Z 10, a pinned plain walk mask of 0xC1) are re-pinned to the synthetic plain (Z 0, 0xFF).
- No tests were deleted. The multi walkability and boat-deck tests assert movement behaviour over a footprint, not what a real multi looks like, so they run on the contrived footprints.
- Maps 0 and 1 keep file indexes 0 and 1, and every stamp writes both, rather than sharing one file index. On a map without files the getters never consult the shared-file cache, and its patch bits would hide one facet's injection from the other.

## Test plan

- [x] `dotnet test -p:SolutionDir=... --filter "Category!=Manual"` with the client env vars pointed at an empty directory: 837 + 1131 passed, 0 skipped, 0 failed
- [x] Mutation check: removing the step-cache clear from `TestTerrain` makes the stale-cache tests fail
- [x] CI green on all platforms

Projects/Server.Tests/Tests/Maps/CanSpawnMobileTileDataTests.cs

@@ -4,23 +4,14 @@ using Xunit;
 namespace Server.Tests.Maps;
 
 /// <summary>
-/// Tests for CanSpawnMobile that require TileData (client files) to be loaded.
-/// These tests will be skipped if client files are not available.
-/// Configure client path via MODERNUO_CLIENT_PATH environment variable or place files at C:\Ultima Online Classic.
+/// Tests for CanSpawnMobile over tile flags, using tiles from the generated test kit.
 /// </summary>
 [Collection("Sequential Server Tests")]
 public class CanSpawnMobileTileDataTests
 {
-    private void SkipIfNoTileData()
-    {
-        Skip.If(!ServerFixture.TileDataLoaded, "TileData not loaded - client files required");
-    }
-
-    [SkippableFact]
+    [Fact]
     public void CanSpawnMobile_FindsLowestValidSurface()
     {
-        SkipIfNoTileData();
-
         var map = Map.Felucca;
         const int x = 1200;
         const int y = 1200;
@@ -44,11 +35,9 @@ public class CanSpawnMobileTileDataTests
         }
     }
 
-    [SkippableFact]
+    [Fact]
     public void CanSpawnMobile_ZRangeSelectsCorrectFloor()
     {
-        SkipIfNoTileData();
-
         var map = Map.Felucca;
         const int x = 1300;
         const int y = 1300;
@@ -72,11 +61,9 @@ public class CanSpawnMobileTileDataTests
         }
     }
 
-    [SkippableFact]
+    [Fact]
     public void CanSpawnMobile_LowCeilingBlocksSpawn()
     {
-        SkipIfNoTileData();
-
         var map = Map.Felucca;
         const int x = 1400;
         const int y = 1400;
@@ -99,11 +86,9 @@ public class CanSpawnMobileTileDataTests
         }
     }
 
-    [SkippableFact]
+    [Fact]
     public void CanSpawnMobile_HighCeilingAllowsSpawn()
     {
-        SkipIfNoTileData();
-
         var map = Map.Felucca;
         const int x = 1450;
         const int y = 1450;
@@ -124,11 +109,9 @@ public class CanSpawnMobileTileDataTests
         }
     }
 
-    [SkippableFact]
+    [Fact]
     public void CanSpawnMobile_BlockerOutsideZRangeIgnored()
     {
-        SkipIfNoTileData();
-
         var map = Map.Felucca;
         const int x = 1475;
         const int y = 1475;
@@ -150,11 +133,9 @@ public class CanSpawnMobileTileDataTests
         }
     }
 
-    [SkippableFact]
+    [Fact]
     public void CanSpawnMobile_WaterSurfaceForSwimmingMob()
     {
-        SkipIfNoTileData();
-
         var map = Map.Felucca;
         const int x = 1550;
         const int y = 1550;
@@ -176,11 +157,9 @@ public class CanSpawnMobileTileDataTests
         }
     }
 
-    [SkippableFact]
+    [Fact]
     public void CanSpawnMobile_WorldItemSurfaceWorks()
     {
-        SkipIfNoTileData();
-
         var map = Map.Felucca;
         const int x = 1650;
         const int y = 1650;