Add alpha parameter to age_scatter for configurable point transparency

[Copilot]

• The title of your PR should briefly describe the change.
• Commit/merge messages to be included in NEWS.md should begin with -.
• Code should follow the tidyverse style guide.
• Documentation should use roxygen2, with Markdown syntax.
• Contributions should include unit tests (using testthat).

Description

Adds configurable point transparency to age_scatter() plots. Previously hardcoded at 0.7.

Changes

Function signatures:

• age_scatter(): Added alpha = 0.7 parameter
• autoplot.pop_data(): Added alpha = 0.7 parameter, passes through to age_scatter()

Documentation:

• Updated roxygen2 docs with parameter description and usage example

Tests:

• Added test verifying alpha parameter accepts custom values

Usage

# Default behavior unchanged
xs_data |> autoplot(type = "age-scatter")

# Custom transparency
xs_data |> autoplot(type = "age-scatter", alpha = 0.3)

For more information see Contributing.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>modify the age scatter function to allow users to change the point alpha</issue_title>
<issue_description>age_scatter <- function(
object,
strata = NULL,
alpha = 0.7, # Add alpha as a parameter
age_var = object |> get_age_var(),
value_var = object |> get_values_var()) {

create default plotting

</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes modify the age scatter function to allow users to change the point alpha #500

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