chore: convert remaining :: docstring blocks to fenced code blocks

[Alberto-Codes]

Context

Story 2.1 established fenced code blocks as the project convention for docstrings (better syntax highlighting, copy buttons in mkdocs-material). Some files still use :: indented-code-block style.

Affected Files

• protocols.py
• __init__.py
• serialization.py
• exceptions.py

Approach

Convert when stories touch these files. Not urgent — purely cosmetic difference in rendered docs.

Origin

Identified in Epic 2 retrospective as low-priority tech debt.