feat(pathfinding): multi-aware mask synthesizer + warm interior cache for house/boat cells (#2479)

## Problem

Houses and boats (multis) were pathed correctly only by **delegation to the slow path**: `StepCache.TryGetMask` returns `Fallthrough_Multi` for any multi-covered cell, and `GetSuccessors` ran `CheckMovement` **8× per cell** (each re-resolving the tile stack via `GetStaticAndMultiTiles`) — a sustained per-step cost near every house/boat. There was also no automated test pinning multi pathfinding.

This branch is the full multi-pathfinding effort in phases on one branch.

## Phase 1 — characterization tests (the oracle)

Implementation-agnostic invariants: a cache-on≡cache-off whole-path invariant, a per-cell sweep vs `CheckMovement` over footprint+halo (incl. destination Z), hand-verified routing (around walls, demolish-reopens, foundation-redesign-honored), classic-house / foundation / boat fixtures, non-vacuity guards. These gate every later phase byte-for-byte.

## Phase 2 — live single-pass synthesizer

`StepProbe.ComputeMultiMaskAt` synthesizes a covered cell's full 8-direction `StepMask` in one pass (the existing surface/step logic over `GetStaticAndMultiTiles` instead of 8× `CheckMovement`). `GetSuccessors` routes `Fallthrough_Multi` cells through it. No new cache, no `.swb` change. **~1.5×**, zero added allocations.

## Phase 3 / 3.1 — warm per-`multiID` interior cache (airtight)

`MultiMaskCache` caches each fixed multi's local-frame `StepMask` for **interior** cells (cell + all 8 neighbours covered → terrain-neighbour-free → position-invariant), keyed by `multiID & 0x3FFF`, built lazily from the MCL. Interior cells become ~20 ns lookups.

The cache is gated on a **per-instance footprint-clean flag** (`BaseMulti.PathInteriorCacheState`): an instance whose whole footprint terrain is below its floor (`maxTerrain < minFloor`) serves from the cache; a **dirty** instance (terrain intrudes — a contrived/GM placement) **degrades to live-synth, never a wrong mask**. This closes a cross-instance soundness gap (the cached mask depends on neighbour terrain too) found in a holistic review. The gate resets whenever the footprint's world-terrain relationship can change — **location, map, or ItemID** (a boat's heading swaps the MCL).

**Boats are cached too.** Their per-`multiID` deck masks are movement-invariant (built once per heading), so a sailing boat never rebuilds them; only the cheap clean-flag rescan repeats per move (and only when pathed near). Narrow existing boats have little interior; wide galleons (`multi.mul`) would gain Castle-class. `HouseFoundation` (per-instance runtime `DesignState`) is the one type that stays on the live path.

## Verification

- `UOContent.Tests` **454/454**, `Server.Tests` **708/708**, 0 failures.
- The Phase-1 oracle (`MultiPathInvariantTests`, cache-on ≡ cache-off) stays **byte-identical** with the synthesizer + interior cache active.
- Tests pin: footprint-cleanliness (clean vs sunk), dirty/cluttered placement degrades to live-synth while still pathing, clean placement serves, and the gate resets on move/ItemID change.

## Performance (modernuo/ModernUO-Benchmarks#8, full-fixture)

Houses at **Green Acres** (flat staff region → clean footprints, the legit-placement case):

| Route | Slow path | Phase 3.1 (interior cache) | Speedup |
|-------|----------:|---------------------------:|--------:|
| `around_a` (29 steps) | 238.3 µs | **49.1 µs** | **4.85×** |
| `around_b` (29 steps) | 224.3 µs | **49.5 µs** | **4.53×** |

~130 of ~167 multi cells/route serve from the cache (~20 ns) vs 37 live-synth. Per-cell, the slow path's 8× `CheckMovement` grows with multi complexity (GuildHouse ~857 ns → Castle ~1,194 ns), the synthesizer is a flat ~780 ns, and the cache serve is ~20 ns — so big/tall multis (and wide galleons) gain most. Identical allocations throughout.

Projects/UOContent.Tests/Fixtures/TestServerInitializer.cs

@@ -57,12 +57,25 @@ internal static class TestServerInitializer
             Server.Network.NetState.Configure();
             TestMapDefinitions.ConfigureTestMapDefinitions();
 
+            // TileData's static cctor short-circuits when running under xUnit
+            // (see Server/TileData.cs:295). Force-load via reflection so LandTable/ItemTable
+            // flags are populated before anything that reads TileData (MultiData, MovementImpl,
+            // CheckMovement). Without this, TileData.MaxItemValue is 0 at MultiData.Configure()
+            // time, causing every MCL tile ID to be masked to 0 and stored as ID=0 in Tiles[x][y].
+            ForceLoadTileData();
+
             // Production runs every static Configure() via AssemblyHandler.Invoke("Configure");
             // the fixture calls a curated subset, so configure the pathfinding singleton here so
             // BitmapAStarAlgorithm.Instance carries its configured MaxSearchNodes before any test
             // calls Find. ServerConfiguration is already loaded above, so the setting resolves.
             BitmapAStarAlgorithm.Configure();
 
+            // Multi component lists (multi.mul / MultiCollection.uop). Production invokes this via
+            // AssemblyHandler.Invoke("Configure"); the curated fixture subset must call it so that
+            // BaseMulti.Components (MultiData.GetComponents) returns real footprints instead of
+            // MultiComponentList.Empty. Required by the Multi pathfinding tests.
+            MultiData.Configure();
+
             World.Configure();
             Timer.Init(0);
             RaceDefinitions.Configure();
@@ -72,12 +85,6 @@ internal static class TestServerInitializer
             World.ExitSerializationThreads();
             DecayScheduler.Configure();
 
-            // TileData's static cctor short-circuits when running under xUnit
-            // (see Server/TileData.cs:295). Force-load via reflection so LandTable/ItemTable
-            // flags are populated; without this, every tile reads as flag=None and
-            // MovementImpl.CheckMovement treats everything as walkable.
-            ForceLoadTileData();
-
             VerifyTrammelTileDataLoaded();
 
             _initialized = true;