Release candidate 0.4.0-rc.1

This release candidate adds typed POCO models for the /v1/exposure/summary/{symbol} endpoint and tightens integration test coverage to reference every declared field.

What's new:

• ExposureSummaryResponse + 5 sub-types (Exposures, Interpretation, HedgingMove, HedgingEstimate, ZeroDte)
• Field-level nullability aligned across all 5 SDK languages
• Integration test references 100% of declared fields

API behaviour clarifications (live + historical probes):

• regime ∈ {positive_gamma, negative_gamma, neutral, undetermined}. The original API doc text omits neutral; the live response includes it when net_gex straddles zero.
• hedging_estimate.*.direction is lowercase "buy"/"sell" on both /exposure/summary and /exposure/zero-dte. The API doc on /exposure/summary shows uppercase but live responses are lowercase. Type definitions reflect actual response.
• as_of_requested exists on /exposure/{gex,dex,narrative} but not on /exposure/summary.

Backwards-compatible: Existing untyped methods continue to work unchanged. Typed return-type annotations (Python TypedDict, TS interface) are non-breaking. Java/C#/Go new POCO types are additive.

This is a release candidate. Final 0.4.0 will follow once VRP and other nested-trap endpoints get the same POCO treatment.