Bump version to 0.5.3

Patch release: docstring fixes (which ship in the package) plus docs-build
hygiene. No API or behavior changes.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: levon003

src/flexeval/__about__.py

@@ -1 +1 @@
-__version__ = "0.5.2"
+__version__ = "0.5.3"