feat(gamma): add typed Gamma API client

[yluoc]

Summary

The SDK had no typed Gamma client. Gamma was parsed ad hoc in three places, so there was no SDK API for market discovery.

• Adds GammaClient, matching the official Python SDK's Gamma calls: markets and events (by ID, slug or URL, plus filtered lists), tags, series, sports, teams, search, comments and public profiles.
• Typed results with a raw JSON copy of each object. Lists skip non-binary markets, as the Python SDK does.
• Paginator gains the paging styles Gamma needs: keyset, page-numbered, and offset pages that count only some rows (top-level comments).
• PositionClient::resolve_market now looks markets up through GammaClient, using /markets/keyset instead of /markets. Its checks and error messages are unchanged.
• Adds scripts/ci_local.sh, which runs this workflow's checks locally, and the missing <cstddef> include in src/evm_rlp.cpp.

Verification

Commands run and their actual results:

• scripts/ci_local.sh --title "feat(gamma): add typed Gamma API client" (WSL Ubuntu, GCC 12, Clang 18):
  • Release, Debug and ASan+UBSan builds: all tests pass.
  • Lint: pass.
  • Clang 18 build: pass.
  • Fuzzers, 60 s each: websocket_messages 440,922 runs, rest_responses 36,331 runs, no crashes.
  • PR title check: pass.
• Mutation checks: each key Gamma rule was broken on purpose (non-binary skip, page-size caps, comment routing, top-level counting, 404 profile), and each made its test fail.

Checks not run and the reason (or none):

• GCC 11 (the ubuntu-22.04 runners): not installed locally, so CI covers it.
• macOS jobs: not possible locally; the Clang 18 build is the closest stand-in.
• test_clob_rate_limit failed intermittently under WSL, passing on every rerun. It checks wall-clock timestamps and isn't touched by this PR.

Compatibility and evidence

Public API, binary compatibility, or release implications (or none):

• New public headers: gamma_client.hpp, gamma_types.hpp, gamma_catalog_types.hpp, gamma_social_types.hpp and gamma_requests.hpp.
• Rebuild required: PageRequest gains page and exhausted, and Page<T> gains counted and has_more.
• Paginator behavior:
  • Cursor paging now reports done() right after its last page.
  • A resume token from a finished keyset or numbered paginator doesn't fetch again.
• PositionClient:
  • It validates gamma_api_url (http or https) when constructed, instead of on the first lookup.
  • Its market parsing is now strict, as elsewhere in GammaClient.

Protocol source: py-sdk 72297f05d6d8 (_internal/actions/gamma.py, models/gamma/*).

Live checks (read-only GETs to the public Gamma API, 2026-10-16):

• Response shapes: these were recorded and became the trimmed test fixtures. Personal data in comment and profile responses is replaced with synthetic values.
• Observed behavior:
  • Gamma caps page sizes: /tags at 100 and /series at 50.
  • It rejects comment offsets above 200 with a 422.
  • /markets/keyset returns the same markets as /markets for open markets, and for closed markets both with and without closed=true.
• Everything else is covered only by local fixture tests. No orders or transactions were sent.

[SebastianBoehler]

Thanks for keeping contributing! Looks good