`data._symcache`, impl a summary `.__repr__()`, avoids `Asset` causality issues

piker/data/_symcache.py

@@ -76,7 +76,7 @@ class SymbologyCache(Struct):
 
     # all asset-money-systems descriptions as minimally defined by
     # in `.accounting.Asset`
-    assets: dict[str, 'Asset'] = field(default_factory=dict)
+    assets: dict[str, Asset] = field(default_factory=dict)
 
     # backend-system pairs loaded in provider (schema) specific
     # structs.
@@ -90,6 +90,18 @@ class SymbologyCache(Struct):
     # provided by the backend pkg.
     mktmaps: dict[str, MktPair] = field(default_factory=dict)
 
+    def pformat(self) -> str:
+        return (
+            f'<{type(self).__name__}(\n'
+            f' .mod: {self.mod!r}\n'
+            f' .assets: {len(self.assets)!r}\n'
+            f' .pairs: {len(self.pairs)!r}\n'
+            f' .mktmaps: {len(self.mktmaps)!r}\n'
+            f')>'
+        )
+
+    __repr__ = pformat
+
     def write_config(self) -> None:
 
         # put the backend's pair-struct type ref at the top