feat: implement Base.circshift and Base.circshift!

[AFeuerpfeil]

Summary

• Implements circshift(arr, shifts), circshift!(arr, shifts) (in-place), and circshift!(dest, src, shifts) for PeriodicArray
• The implementation materializes the new unit cell by evaluating src[k - s] for each index k, correctly applying the boundary map (e.g. Bloch phase factors) for elements that wrap across cell boundaries — unlike a naive circshift(parent(arr), shifts) which would ignore the map at the seam
• Multiple specific method signatures are used to resolve all method ambiguities with Base (verified by Aqua)
• Fixes a linter-introduced regression where Vararg{Int, N} was widened to Vararg{Integer, N} in getindex/setindex!, which had reintroduced 4 method ambiguities

Test plan

• circshift(a, s)[i] == a[i - s] for all integers i, verified across many shifts (including wrapping cases) for both identity and non-trivial maps
• The "seam" element circshift(a, 1)[1] == a[0] == f(data[end], -1) is correctly computed with the map applied
• Roundtrip: parent(circshift(circshift(a, s), -s)) == parent(a) for all shift values
• circshift!(dest, src, shifts) (3-arg) and circshift!(arr, shifts) (in-place) both tested
• All 956 tests pass, Aqua reports 0 method ambiguities

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 89.28571% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/PeriodicArrays.jl	89.28%	3 Missing ⚠️

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