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/MultiEdgeCaseTests.cs

@@ -1,5 +1,6 @@
 using Server.Engines.Pathing.Cache;
 using Server.Items;
+using Server.Tests.Terrain;
 using Xunit;
 
 namespace Server.Tests.Pathfinding;
@@ -8,16 +9,16 @@ namespace Server.Tests.Pathfinding;
 public class MultiEdgeCaseTests
 {
     private const int MapId = 1; // Trammel
-    private const int GuildHouseId = 0x74;
+    private const int GuildHouseId = TestMultis.GuildHouse;
 
-    // GuildHouse placement (mirrors MultiMaskSynthesisTests) — open Trammel ground.
+    // GuildHouse placement (mirrors MultiMaskSynthesisTests) — open ground.
     private const int HouseX = 1480;
     private const int HouseY = 1620;
 
-    // Open water in the south-Britain bay (mirrors BoatPathTests).
-    private const int BoatMultiId = 0x0; // SmallBoat North heading
-    private const int WaterX = 1450;
-    private const int WaterY = 1770;
+    // Open water: the middle of the DeepSea prefab.
+    private const int BoatMultiId = TestMultis.SmallBoatNorth;
+    private static int WaterX => Prefabs.DeepSea.Anchor.X;
+    private static int WaterY => Prefabs.DeepSea.Anchor.Y;
     private const sbyte DeckZ = 0; // boat deck floor tiles stand at world Z 0 (not the water avgZ)
 
     /// <summary>
@@ -25,10 +26,9 @@ public class MultiEdgeCaseTests
     /// <c>GetStaticAndMultiTiles</c> yields tiles from BOTH multis; the synthesizer must still
     /// agree with CheckMovement everywhere over the union footprint + halo.
     /// </summary>
-    [SkippableFact]
+    [Fact]
     public void OverlappingMultis_SynthesizerMatchesCheckMovement()
     {
-        TileDataRequirement.SkipIfMissing();
         StepCache.Instance.Clear();
         var map = Map.Maps[MapId];
 
@@ -76,10 +76,10 @@ public class MultiEdgeCaseTests
     /// blocks the rest — that vacuity is a fixture concern, not a synthesizer divergence (the
     /// synthesizer agrees with the oracle at every direction either way).
     /// </remarks>
-    [SkippableFact]
+    [Fact]
     public void BoatOverWater_SynthesizerMatchesCheckMovement()
     {
-        TileDataRequirement.SkipIfMissing();
+        Prefabs.DeepSea.AssertPresent();
         StepCache.Instance.Clear();
         var map = Map.Maps[MapId];
 
@@ -100,10 +100,9 @@ public class MultiEdgeCaseTests
     /// re-registration pattern HouseFoundation uses on commit) and assert the synthesizer reads
     /// the LIVE, post-redesign <c>Components</c> — i.e. it matches CheckMovement on the NEW shape.
     /// </summary>
-    [SkippableFact]
+    [Fact]
     public void RedesignedFoundation_SynthesizerMatchesNewFootprint()
     {
-        TileDataRequirement.SkipIfMissing();
         StepCache.Instance.Clear();
         var map = Map.Maps[MapId];
 
@@ -117,7 +116,7 @@ public class MultiEdgeCaseTests
             // Internalize() fires Map.OnLeave (removes the OLD footprint's registration); we swap
             // the MCL and MoveToWorld back, firing Map.OnEnter -> AddMulti against the new shape.
             foundation.Internalize();
-            foundation.Redesign(MultiData.GetComponents(0x7A)); // Tower footprint (different shape)
+            foundation.Redesign(MultiData.GetComponents(TestMultis.Tower)); // Tower footprint (different shape)
             foundation.MoveToWorld(loc, map);
 
             MultiTestSupport.AssertSynthesizerMatchesCheckMovement(foundation, map);