feat: add auto-pagination iterators for list endpoints

[harley-poly]

summary

adds iterate() async generators that transparently walk all pages so callers no longer hand-roll cursor/offset logic:

• offset-paginated resources: events.iterate(), markets.iterate(), series.iterate() (page until a short page is returned).
• cursor-paginated feed: portfolio.iterateActivities() (follow nextCursor until eof or an empty cursor).
• shared helpers live in src/pagination.ts (paginateOffset / paginateCursor).

positions are intentionally not iterated (the endpoint returns the full set in one response).

test plan

• pnpm lint (biome)
• pnpm typecheck / pnpm build
• pnpm test (135 passed; new tests/pagination.test.ts covers offset paging until short/empty page and cursor paging until eof/no-next-cursor)

---

Note

Low Risk
Additive client-side helpers and new optional APIs; existing list/activities calls are unchanged, with tests covering termination edge cases.

Overview
Adds lazy auto-pagination so callers can stream full list results without manual offset / cursor loops.

New shared helpers in src/pagination.ts: paginateOffset (stops on a short or empty page; clamps non-positive page size) and paginateCursor (follows nextCursor until eof or missing cursor). events, markets, and series gain iterate(params?, pageSize?); portfolio gains authenticated iterateActivities(params?). README documents for await usage. tests/pagination.test.ts covers multi-page offset iteration, empty-page stop, and cursor/eof behavior.

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