Fix copy+paste of PegCurveGenerator always producing BrickCurveGenerator

[Copilot]

CurveGenerator.Clone() was hardcoded to construct a BrickCurveGenerator regardless of the actual subtype, so copying a Peg Line Generator and pasting it would silently produce a Brick Line Generator instead.

Changes

• CurveGenerator: Replace Clone() with a protected CloneTo(CurveGenerator) helper that copies BezierPath and base properties — shared by both subclasses.
• PegCurveGenerator: Add Clone() override that constructs the correct PegCurveGenerator type.
• BrickCurveGenerator: Add Clone() override that constructs the correct BrickCurveGenerator type.

// Before — in CurveGenerator, always wrong type:
public override object Clone()
{
    var copy = new BrickCurveGenerator(Level); // bug: hardcoded
    base.CloneTo(copy);
    copy.BezierPath = BezierPath.Clone();
    return copy;
}

// After — each subclass owns its clone:
// PegCurveGenerator:
public override object Clone()
{
    var copy = new PegCurveGenerator(Level);
    base.CloneTo(copy);
    return copy;
}

Original prompt

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This section details on the original issue you should resolve

<issue_title>Copy+pasting Peg Line pastes Brick Line instead</issue_title>
<issue_description>When copying an unapplied Peg Line Generator, pasting it results in an identical Brick Line Generator instead. Not sure what's causing it but definitely a bit of a pain.

</issue_description>

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

• Fixes Copy+pasting Peg Line pastes Brick Line instead #30

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