Fix- pen tool causing jitter at the endpoints

[Chetansahney]

Fixes #3912
(the pen merging issue at the endpoints) with reference to the conversation:

https://discord.com/channels/731730685944922173/731738914812854303/1483829916096139377
&
https://discord.com/channels/731730685944922173/731738914812854303/1481800685107544165

Any feedbacks are appreciated.

[gemini-code-assist]

Summary of Changes

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This pull request addresses a specific pen tool issue related to merging paths at their endpoints by refining the logic for identifying extendable paths. Additionally, it significantly enhances the user experience for editing fundamental vector shapes like circles, arcs, and spirals by introducing linked collinear manipulators, allowing for more intuitive and unified adjustments of these geometric forms.

Highlights

• Pen Tool Endpoint Merging Fix: The should_extend utility function has been refactored to directly identify the closest anchor endpoints for path merging, resolving an issue where the pen tool had difficulties merging at endpoints.
• Improved Vector Shape Editing: The circle, arc, and spiral node generators now explicitly add colinear_manipulators to their generated vector shapes. This enhancement links adjacent handles, making these shapes easier to edit and manipulate as cohesive units, especially for curves.

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[gemini-code-assist]

Code Review

The pull request refactors the should_extend function with an inline implementation for finding the closest point, though a review comment suggests encapsulating this logic for better readability and maintainability. Additionally, the circle, arc, and spiral shape generation functions were updated to add colinear_manipulators to the generated Vector objects, linking all handles for circles and linking handles for adjacent cubic bezier segments in arcs and spirals. Review feedback suggests refactoring the duplicated handle linking logic in arc and spiral into a reusable function, adding comments to explain the is_cubic() check, and considering HandleId::new for creating handle IDs for consistency and safety.

[cubic-dev-ai]

No issues found across 2 files