Improve list join performance

[liufengyun]

Improve list join performance

No API change, only performance improvement.

Summary

The change removes repeated copying of the accumulated prefix in List.join, using native array joining after rendering each element once.

Backend	Separator	Elements	Before (ms)	After (ms)	Change
Ruby	""	200,000	7,520.2	242.3	31.0× faster
Ruby	","	200,000	16,140.7	250.2	64.5× faster
Python	""	200,000	271.9	281.7	3.6% slower
Python	","	200,000	8,357.1	262.4	31.9× faster
JavaScript	""	200,000	41.6	13.0	3.2× faster
JavaScript	","	200,000	63.1	25.3	2.5× faster

Checklist

• Added / updated tests under tests/pos/ or tests/warn/
• Docs updated if the change affects user-visible behavior
• Update CHANGELOG.md
• A JIP is required for language change
• All commits are signed off (why?)

How to sign off commits

Use git commit -s to add the Signed-off-by line automatically:

To add a sign-off to the last commit retroactively:

git commit --amend -s --no-edit

To add sign-off to the last 3 commits:

git rebase --signoff HEAD~3

Security impact

No

Compatibility impact

No

• Source compatibility
  • syntax parsing
  • type checking
• SAST compatibility
  • forward compatibility: new libraries can be used by old compiler
  • backward comopatibility: old libraries can be used by new compiler
• Standard Library compatibility:
  • forward compatibility: new code can work with old stdlib
  • backward compatibility: old code work with the new stdlib
• Runtime Library compatibility:
  • forward compatibility: new code can work with old runtime
  • backward compatibility: old code work with the new runtime
• Build tool
  • Build spec compatibility: old projects continue to build
  • Joy package compatibility: old .joy can be consumed by new build tool

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