Feat/intersection set operations

[DagmawiKK]

This PR adds intersection set operations endpoint to compute common atoms across multiple source namespaces using a shared $x variable, writing results to a single target namespace.

Key Changes

• New endpoint + intersection_transform helper.
• Permission checks: read on all sources, write on target.
• Unit + integration tests for success, invalid input, unauthorized.
• Frontend Intersection page

Behavior

(transform
    (, (source1 $x) (source2 $x) ...)
    (, (target $x))
)

Notes

• Requires ≥2 sources and exactly 1 target (backend validation).
• Intersection with identical sources degenerates to identity.
• Consistent with existing composition implementation.

Type of Change

• 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 not work as expected)
• This change requires a documentation update

Checklist

• I have read the Contributing Guidelines.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• Any dependent changes have been merged and published in downstream modules.

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