fix: remove dead builder methods and CLI output typo

[izar]

Summary

• Dead methods deleted from _FlowDefaultsBuilder: _color, display_name, _label, _shape, _safeset (lines 414–433 in the pre-fix file). These were orphaned copies of Element-level methods accidentally left inside the @dataclass during the Pydantic migration. They were never called on the builder — the real implementations live on the element classes. Additionally, _label called wrap(...) which is not imported in pytm.py (it's imported in element.py), meaning any invocation would have raised NameError. Deletion resolves the latent bug.
• _list_elements typo fixed: print("\nAtributes:") → print("\nAttributes:"). No test asserted on the misspelled string.
• Element added to _CLASS_REGISTRY: _describe_classes iterated only subclasses of Element, not Element itself, so --describe Element errored with "No such class to describe: Element". One-line fix restores the documented CLI behaviour.

Validation

python3 tm.py --list-elements   # "Attributes:" printed correctly; no NameError
python3 tm.py --describe Element  # works, shows current Pydantic fields
uv run pytest tests/ -x -q      # 243 passed
git diff --stat HEAD~1           # pytm/pytm.py | 23 +++...--- (no README touches)

[izar]

I'm merging as a reviewer of AI-generated code...