feat: add auto-pagination iterators for list endpoints

[harley-poly]

summary

adds iterate() helpers 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.iterate_activities() (follow nextCursor until eof or an empty cursor).
• shared helpers live in polymarket_us/pagination.py; both sync (Iterator) and async (AsyncIterator) variants are provided.

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

test plan

• ruff check / ruff format --check
• mypy polymarket_us
• pytest (125 passed; new tests/test_pagination.py covers offset paging until short/empty page, cursor paging until eof/no-next-cursor, and async variants)

---

Note

Low Risk
Additive client-side helpers and documentation; no changes to signing, orders, or server contracts beyond extra list requests when iterating.

Overview
Adds auto-pagination so callers can stream full list results without manual offset/cursor handling. Shared logic in polymarket_us/pagination.py covers offset pages (stop on short/empty page, clamp page_size to at least 1) and cursor pages (follow nextCursor until eof or missing cursor), with sync and async variants.

iterate() on events, markets, and series walks offset-paginated lists (optional filters + page_size). portfolio.iterate_activities() walks authenticated activity history via cursor. README documents sync/async for usage.

Tests in tests/test_pagination.py cover multi-page offset, empty pages, cursor/eof, and async mirrors. test_auth now builds a valid 64-byte base64 secret for the Ed25519 key-length test.

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