Address PR #238 review-thread feedback in drawing example and interactive input helper

[Copilot]

This PR applies all requested updates from the linked review thread on PR #238, with no unrelated scope expansion. It tightens drawing schema validity, aligns discriminant conventions, and removes two renderer/interactive edge-case failure points.

• Schema tightening + consistency

  • Restricts Style.line_style to Literal["solid", "dashed", "dotted"] instead of free-form str.
  • Aligns unknown-case discriminant with existing examples by changing UnknownText.type to Literal["Unknown"].
  • Updates Arrow docstring to match implemented behavior (arrow head at end).

• Renderer hardening

  • Replaces dash-pattern direct indexing with a safe fallback to solid line when line_style is unexpected.

• Lint-safe optional readline import

  • Marks optional readline import as intentionally unused to avoid F401 noise while preserving side-effect behavior.

# Before
dash=dash_pattern[getattr(arrow.style, "line_style", None) or "solid"]

# After
dash=dash_pattern.get(getattr(arrow.style, "line_style", None) or "solid", "")

[robgruen]

I made these changes in @gvanrossum 's fork and so they're now in #238