Add fast __deepcopy__ for FT to avoid slow generic object fallback

[AmiraliOmidvar]

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Related Issue
N/A (performance optimization)

Proposed Changes
Add a fast deepcopy implementation for FT (Fast Tag) to avoid the slow generic object fallback. The implementation directly copies the five core fields (tag, children, attrs, void_, listeners_) while delegating unknown attr/child value types to deepcopy itself, so user-defined deepcopy methods are still respected. This improves performance when copying FT components.

Types of changes
What types of changes does your code introduce? Put an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist
Go over all the following points, and put an x in all the boxes that apply:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.
• I am aware that this is an nbdev project, and I have edited, cleaned, and synced the source notebooks instead of editing .py or .md files directly.

Additional Information
Tests added in nbs/api/00_core.ipynb cover:

• Basic deepcopy with identity verification
• Nested structure mutation isolation
• Void element preservation
• Custom deepcopy delegation
• fast_app() header/footer deepcopy

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