fix: Fixes pathfinding prebake and pathfinding multi-fallthrough. (#2478)

## Summary

Fixes the pathfinding step-cache (`.swb`) prebake so it bakes **once** and skips when a valid cache already exists, instead of re-baking on every boot. The root cause was the staleness fingerprint hashing mutable in-memory tile data rather than the on-disk files. This PR makes the fingerprint a pure function of the client data files and separates dynamic multis (houses/boats) from the static cache.

> This branch builds on the `ConfigurePrompts` first-boot-prompt unification (commit `5df8d0bd`, also included here) — that commit accounts for the `ServerConfiguration.cs` and `dev-docs/server-lifecycle.md` changes in the diff.

## The bug

With `pathfinding.prebakeMaps` set, the cache re-baked on **every** boot. The `.swb` staleness fingerprint hashed the live `TileData.LandTable`/`ItemTable` flags, which the server patches at runtime (`ItemFixes`, `LOSBlocker`, `PotionKeg`, `CTF`) at nondeterministic lifecycle points (Initialize-phase methods share a priority; static ctors fire lazily). So a fingerprint stamped at runtime (`[PathBake`) never matched the one recomputed during startup `Initialize()`, and the cache rebaked every time.

## Changes

**1. Fingerprint the files, not the in-memory tables** (`fix`)
Hash `tiledata.mul` (cached, computed once) plus the per-map `.mul`/`.uop` files — never the runtime-mutated `TileData` tables. The fingerprint is now lifecycle-stable. Existing `.swb` files rebake once after deploy, then stay stable.

**2. Compute the fingerprint once per boot** (`refactor`)
`Configure()`'s `AutoLoadAtStartup()` already opens and fingerprint-validates a reader for every up-to-date `.swb`. `Initialize()` now skips baking any map that already has an open reader (`StepCache.HasLazyReader`) instead of recomputing the fingerprint a second time.

**3. Bake static-only; route multis to the live path** (`refactor`)
Multis (houses/boats) are dynamic, so they're no longer baked into the static chunk cache — they were tagged with `BuiltMultisVersion`, a non-persisted session counter, which made persisting them unsafe (false matches / wasted re-bakes).
- Chunks bake land + `statics.mul` only.
- At query time, any cell whose sector (or its 1-cell halo) contains a multi routes to `Fallthrough_Multi` → the existing live, multi-aware `CheckMovement` path. The halo prevents a cell proposing a walkable edge into a neighbouring wall; interior (multi-free) cells pay one sector lookup.
- Adds `Sector.HasMultis` (one engine accessor); `.swb` format → v9 (rejects old multi-baked files); new `Fallthrough_Multi` telemetry.
- Behaviour-preserving: multis use the same live path the engine used before the cache existed.

**4. Comment polish** (`style`) — no behaviour change.

## Testing

All green: **92** pathfinding (incl. a new fingerprint-stability test and a multi-halo fallthrough test), **423** UOContent, **708** Server.

## Follow-ups (not in this PR)

- **Background bake worker** — make `[PathBake` and the boot prebake non-blocking (game thread serves tile reads to an off-thread worker).
- **Per-multi MCL cache** — cache walkability in each multi's own frame (keyed by multiID, movement-invariant) so houses/boats get a fast path instead of the live fallback.
- **House-pathfinding equivalence tests** — the one area not yet covered by a dedicated automated test; multi pathing is currently correct by delegation to the live path.

Projects/UOContent.Tests/Tests/Engines/Pathing/StepCacheFileTests.cs

@@ -114,6 +114,49 @@ public class StepCacheFileTests
         Assert.Equal(0, cache.OpenLazyReaderCount);
     }
 
+    /// <summary>
+    /// HasLazyReader is the boot prebake's skip predicate (PathCacheCommands.Initialize): a map
+    /// with an open, fingerprint-valid reader needs no bake. Lock the open/clear contract.
+    /// </summary>
+    [Fact]
+    public void HasLazyReader_TracksOpenAndClear()
+    {
+        var cache = StepCache.Instance;
+        cache.Clear();
+
+        var map = Map.Maps[1];
+        Assert.NotNull(map);
+        Assert.False(cache.HasLazyReader(map.MapID));
+
+        // Build + save a chunk so there's a valid .swb to open.
+        cache.MissPromotionThreshold = 1;
+        Span<sbyte> surfZ = stackalloc sbyte[16];
+        Assert.True(StepProbe.ComputeStandableSurfaceZs(map, 1500, 1600, surfZ) > 0);
+        cache.TryGetMask(map, 1500, 1600, surfZ[0]);
+
+        var path = Path.Combine(Path.GetTempPath(), $"step-cache-haslazy-{Guid.NewGuid():N}.swb");
+        try
+        {
+            Assert.True(cache.SaveToFile(path, map.MapID) > 0);
+            cache.Clear();
+            Assert.False(cache.HasLazyReader(map.MapID));
+
+            Assert.True(cache.TryOpenLazyReader(path, map.MapID));
+            Assert.True(cache.HasLazyReader(map.MapID)); // open → true
+
+            cache.Clear();
+            Assert.False(cache.HasLazyReader(map.MapID)); // clear closes the reader → false
+        }
+        finally
+        {
+            cache.Clear();
+            if (File.Exists(path))
+            {
+                File.Delete(path);
+            }
+        }
+    }
+
     [Fact]
     public void TryOpenLazyReader_BadMagic_ReturnsFalse()
     {