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/UOContent.Tests/Tests/Engines/Pathing/Multi/MultiMaskSynthesisTests.cs

@@ -1,4 +1,5 @@
 using Server.Engines.Pathing.Cache;
+using Server.Tests.Terrain;
 using Xunit;
 
 namespace Server.Tests.Pathfinding;
@@ -7,14 +8,13 @@ namespace Server.Tests.Pathfinding;
 public class MultiMaskSynthesisTests
 {
     private const int MapId = 1;            // Trammel
-    private const int GuildHouseId = 0x74;  // static house: walls, door aperture, floor
+    private const int GuildHouseId = TestMultis.GuildHouse;  // static house: walls, door aperture, floor
     private const int PlaceX = 1480;
     private const int PlaceY = 1620;
 
-    [SkippableFact]
+    [Fact]
     public void ComputeMultiMaskAt_MatchesCheckMovement_OverFootprintAndHalo()
     {
-        TileDataRequirement.SkipIfMissing();
         StepCache.Instance.Clear();
         var map = Map.Maps[MapId];
         map.GetAverageZ(PlaceX, PlaceY, out _, out var z, out _);