geometry: unify generate_points and init_points in Arc

Nobody has claimed this yet.

## Source

`manim/mobject/geometry/arc.py`, line 345 (as of HEAD ):

```
def generate_points(self) -> None:
self._set_pre_positioned_points()
self.scale(self.radius, about_point=ORIGIN)
self.shift(self.arc_center)

# Points are set a bit differently when rendering via OpenGL.
# TODO: refactor Arc so that only one strategy for setting points
# has to be used.
def init_points(self) -> None:
self.set_points(
Arc._create_quadratic_bezier_points(
angle=self.angle,
start_angle=self.start_angle,
n_components=self.num_components,
),
)
self.scale(self.radius, about_point=ORIGIN)
self.shift(self.arc_center)
```

## Problem

The `Arc` class exposes two methods that compute and position points in slightly different ways: `generate_points()` and `init_points()`. The duplication exists because the Cairo and OpenGL renderers historically needed different point-setting strategies. The trailing operations (`scale` and `shift`) are identical between the two.

Maintaining both methods doubles the surface for regressions — any change to how arc points are positioned has to be made in both places, and it is easy to update one and forget the other.

## Suggested approaches

- Extract the common scale/shift tail into a helper, and have both methods delegate the actual point computation to a single strategy selected by renderer type.
- Investigate whether the Cairo/OpenGL distinction is still necessary, or whether the codebase has converged enough that a single point-setting routine suffices.
- Audit related geometry classes (subclasses of `Arc`, and other shapes in `manim/mobject/geometry/`) for the same duplication pattern.

Start in manim/mobject/geometry/arc.py around line 345 and compare Arc.generate_points() with init_points(). Investigate whether the Cairo/OpenGL point-setting distinction is still required, then inspect Arc subclasses and related shapes in manim/mobject/geometry/ for the same pattern. Done means reducing duplicated positioning logic without changing Arc behavior and accounting for any necessary renderer-specific strategy.

Written by the indexing model from the issue text.