feat: batch 1-3 API updates — spec alignment, fee_cost, subaccounts, _fp fields

[pbeets]

Summary

• Implements Kalshi API changelog items from Jan 22 – Jan 29, 2026 (batches 1-3)
• Aligns model field types with the official Kalshi API spec — promotes count_fp, yes_price_fixed/no_price_fixed/yes_price_dollars/no_price_dollars, and fee_cost from Option<String> to String where the spec marks them required
• Adds fee_cost (exchange fee as fixed-point dollar string) to Fill, Settlement, and WebSocket FillData
• Adds subaccount filter to GetSettlementsParams and GetFillsParams
• Updates SubaccountBalance to use String balance (fixed-point dollars) and required updated_ts, matching current API
• Adds order group trigger/limit endpoints, account API limits endpoint, and OrderGroupUpdates WebSocket channel
• Adds #[deprecated] attributes to Kalshi-deprecated items (legacy fields, old methods)
• Adds _fp fixed-point fields across REST and WebSocket response types
• Updates examples to use fixed-point dollar fields and display fee_cost

Breaking changes

• SubaccountBalance::balance: i64 → String; updated_ts: Option<i64> → i64; balance_dollars() removed
• Fill::count_fp, Fill::yes_price_fixed, Fill::no_price_fixed, Fill::fee_cost: Option<String> → String
• Fill::created_time: String → Option<String>
• Settlement::yes_count_fp, Settlement::no_count_fp, Settlement::fee_cost: Option<String> → String
• Trade::count_fp, Trade::yes_price_dollars, Trade::no_price_dollars: Option<String> → String
• See CHANGELOG.md ### Changed section for full list

Test plan

• cargo test --lib — 186 tests pass
• cargo check --examples — all examples compile
• cargo doc --no-deps — docs generate cleanly
• Manual verification against live Kalshi API responses