Feat/set operations simplify

[natnael-eyuel]

This PR introduce a simplified, page-driven set operations UI foundation.

Key Changes

• New: ops API for set operations
• New: state with in-place mutations and default compose fallback
• New: item list for patterns/templates
• New: expression preview

Notes

• Page-driven configs: each page supplies its own composeExpression and canRun.
• State: arrays mutate in-place to preserve input focus and values on add/remove.
• No backend changes included in this PR.

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.