bun: persist global data in Scoop persist dir and migrate legacy ~/.bun

[A-Words]

Closes #7696

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Summary

Fix bun manifest so Bun data is persisted under Scoop persist directory, and add safe migration for legacy %USERPROFILE%\.bun.

Changes

• Add env_set:
  • BUN_INSTALL=$persist_dir
• Add env_add_path:
  • "bin"
• Add persist:
  • "bin", "install"
• Extend pre_install:
  • migrate %USERPROFILE%\.bun to $persist_dir when persist dir does not exist
  • if both paths exist, skip migration and print warning to avoid overwrite
• Keep existing AVX2 selection / download / hash / autoupdate logic unchanged

Validation

• bucket/bun.json parses successfully (ConvertFrom-Json)
• scoop cat .\bucket\bun.json shows expected new fields
• bin/test.ps1 could not be executed locally due to missing module: BuildHelpers

[github-actions]

All changes look good.

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bun

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