fix(asset-bridge): an empty catalog is an absent one on every family, not just the tree

Phase 7 found this on the tree family and fixed it there. It was inline in THREE
places: the body catalogue (phase 3), statics and land (phase 5), and the tree.
`expected != null` treats "" as a real fingerprint, so a caller that serialises a
missing value as an empty string has EVERY fetch refused -- with a sentence that
names no catalog at all ("catalog  is now 8159778b"), which reads as a shard
fault rather than a caller one.

All three now go through one BridgeAssets.CatalogMismatch. Three copies of a
comparison are three chances for the next family to get it wrong in a way only a
differently-written client would ever reveal.

BridgeLeases keeps its own `expected != null` and is deliberately untouched:
there the value is a world property, where an empty string is a legitimate thing
to expect.

Verified against a live shard on a stock ServUO install, every family asked three
ways -- with a real catalog, with the field absent, and with an empty string:

  cliloc.table walk                     67,496 rows, 12 pages
  body manifest / fetch                 1,095 rows; ok all three ways
  static + land fetch                   ok all three ways
  static/land carry their OWN catalog   art 66a112c1 vs body 323f284f
  a cross-family catalog                refused 422
  tree manifest / fetch                 141 files incl. BOTH empty ones, all three ways
  empty files carry a VALID gzip member 2 rows gunzip to 0 bytes
  a STALE catalog                       still refused on body, static and tree

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016wDDVXWMDz82WqE1i969r4

overlay/Scripts/Custom/Bridge/BridgeTree.cs

@@ -421,12 +421,10 @@ namespace Server.Custom.Bridge
             List<TreeFile> files = Enumerate();
             string fingerprint = FingerprintOf(files);
 
-            // `IsNullOrEmpty`, not `!= null`. A caller that has no fingerprint to assert sends
-            // the field absent OR empty depending on how its own client serialises a missing
-            // value, and the two must mean the same thing — an empty string compared against a
-            // real id refuses every fetch, with a sentence that names no catalog at all
-            // ("catalog  is now 8159778b"). Found by a probe that passed one.
-            if (!String.IsNullOrEmpty(expected) && expected != fingerprint)
+            // Shared with every other family on this plane, because an absent fingerprint and an
+            // empty one have to mean the same thing here and there — see
+            // `BridgeAssets.CatalogMismatch` for what treating them differently costs.
+            if (BridgeAssets.CatalogMismatch(expected, fingerprint))
             {
                 // The tree moved between the manifest and this fetch. The same refusal the
                 // catalogue makes for a patched client, and for the same reason: these keys were