legacy: extract LegacyArchives into legacy/archives.py, add ArchivesInterface, refs #9556

[mr-raj12]

Moves the Borg 1.x archive list management out of Archives and into LegacyArchives, refs #9556.

• LegacyArchives extracted from Archives into legacy/archives.py
• Archives in manifest.py is now 2.x only as all self.legacy branches removed
• ArchivesInterface added to manifest.py as a @runtime_checkable Protocol as mypy/pyright will catch drift between the two implementations
• Manifest.__init__ picks LegacyArchives or Archives based on repository type via lazy imports

No logic changes. borg transfer --from-borg1 and borg serve (v1 clients) keep working through borg.legacy.archives.

Refs #9556

Checklist

• PR is against master
• New code has tests
• Tests pass
• Commit message references related issue

[codecov]

Codecov Report

❌ Patch coverage is 48.07692% with 108 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.97%. Comparing base (de0f7ac) to head (bf756da).
⚠️ Report is 21 commits behind head on master.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/borg/legacy/archives.py	47.97%	67 Missing and 10 partials ⚠️
src/borg/manifest.py	48.33%	9 Missing and 22 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9639      +/-   ##
==========================================
- Coverage   83.31%   82.97%   -0.35%     
==========================================
  Files          89       90       +1     
  Lines       15549    15673     +124     
  Branches     2343     2374      +31     
==========================================
+ Hits        12955    13005      +50     
- Misses       1838     1889      +51     
- Partials      756      779      +23     

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