Skip redundant re-validation of constructor kwargs

HasTraits.__init__ validated every trait kwarg twice: once via setattr in the
fast loop, then again via _cross_validate + set_trait. The second pass is only
needed for traits that actually have a cross-validator (@Validate handler or a
deprecated _<name>_validate method); for the common case with none it re-ran
validate() on an already-validated value.

The second loop now guards on the same condition _cross_validate itself uses
(key in self._trait_validators or a _<name>_validate attribute exists). For
traits without a cross-validator it records the already-stored (possibly
coerced) value for the notification instead of re-validating, so notification
payloads are byte-for-byte identical.

Measured (Python 3.11): instantiation with kwargs and no cross-validators
~1.2-1.4x faster.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VaKDJ3fpGf7anQeYeBsJbk

Author: claude

traitlets/traitlets.py

@@ -1394,9 +1394,17 @@ def ignore(change: Bunch) -> None:
             # notify and cross validate all trait changes that were set in kwargs
             changed = set(kwargs) & set(self._traits)
             for key in changed:
-                value = self._traits[key]._cross_validate(self, getattr(self, key))
-                self.set_trait(key, value)
-                changes[key]["new"] = value
+                # Only re-run the (relatively expensive) cross-validation +
+                # set_trait pass for traits that actually have a cross-validator.
+                # For the common case with none, the value stored by the fast
+                # loop above is already fully validated; we just need to record
+                # the (possibly coerced) stored value for the notification.
+                if key in self._trait_validators or hasattr(self, f"_{key}_validate"):
+                    value = self._traits[key]._cross_validate(self, getattr(self, key))
+                    self.set_trait(key, value)
+                    changes[key]["new"] = value
+                else:
+                    changes[key]["new"] = getattr(self, key)
             self._cross_validation_lock = False
             # Restore method retrieval from class
             del self.notify_change