[TexasCoding]

Closes #540

Nightly strict spec-drift found two perps drifts after the upstream perps OpenAPI re-pin:

1. New endpoint GET /margin/markets/orderbooks → markets.bulk_orderbooks(*, tickers) on sync and async perps clients. tickers is required (1–100), serialized comma-joined (style: form, explode: false); returns list[MarginOrderbook]. Public GET (no auth), retried per existing GET policy. Missing/empty per-item ticker raises KalshiError.
2. MarginMarket.funding_invalid_premium_handling — spec-required Literal["dampen", "exclude"]. Breaking for code that constructs MarginMarket without it (live list/get responses are unaffected). Migration note added to docs/migration.md; next release is a major bump.

Specs: specs/perps_openapi.yaml re-pinned; core OpenAPI 3.34.0 unchanged; scripts/generate.py produced no _generated/ diff. Optional MarginMarket.market_id remains additive-optional drift (warning only, not a gate failure).

Blast radius (GitNexus unavailable — manual)

• MarginMarket: consumed by PerpsMarketsResource.list/get (+ async), contract map entry, tests/perps/test_markets.py, tests/integration/test_perps_markets.py, docs.
• New GetMarginMarketOrderbooksResponse, _orderbook_from_item, _bulk_orderbooks_params, bulk_orderbooks: only called by the new methods. _parse_orderbook reused unchanged; _join_tickers reused from kalshi.resources._base.
• Endpoint registered in tests/_contract_support.py METHOD_ENDPOINT_MAP and PERPS_CONTRACT_MAP.

Gate (local)

• uv run ruff check . — pass
• uv run mypy kalshi/ — pass (160 files)
• uv run pytest tests/ --ignore=tests/integration -q — 4693 passed
• uv run pytest tests/test_contracts.py -q -W error::UserWarning (nightly strict) — 850 passed