refactor: extract shared pagination helper from duplicated boilerplate

[va932555-art]

Summary

Extracts the duplicated "clamp limit, skip start, break at count" pagination loop into a shared generic paginate() function in a new pagination.rs module.

Changes

• New dongle-smartcontract/src/pagination.rs — generic paginate<T: Clone>(env, items, start, limit) -> Vec<T> with MAX_PAGE_LIMIT = 100
• Updated lib.rs — added pub mod pagination;
• Updated featured_registry.rs — list_featured_projects now calls paginate() on IDs, then looks up projects
• Updated collection_registry.rs — both list_collections and list_collection_projects now call paginate()

What was removed

~30 lines of identical pagination boilerplate across 3 functions, replaced with 3 one-line paginate() calls.

Fixes #336

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