0.18.0

This release adds multiple selection support to the Menu component.

When passing multiple=True to RichToolkit.ask() or Menu(), the menu
switches to multi-select mode:

• Uses square characters (■/□) instead of circles (●/○)
• Space toggles the checked state of the current item
• Arrow keys / j/k move the cursor independently of checked state
• Enter confirms the selection (requires at least one checked item)
• Returns a list of selected values instead of a single value

selected = app.ask(
    "Which languages do you use?",
    options=[
        {"name": "Python", "value": "python"},
        {"name": "JavaScript", "value": "javascript"},
        {"name": "Rust", "value": "rust"},
    ],
    multiple=True,
)
# selected is e.g. ["python", "rust"]

Works with filtering (allow_filtering=True) and all existing styles
(tagged, bordered, etc.). Raises ValueError if combined with inline=True.

This release was contributed by @patrick91 in #46