feat(us): US market APIs + DC-region guards

[hogan-yuan]

What

Adds 14 US-market-only API methods across the SDK, and back-ports the AP-only region guard from the Rust SDK (openapi#554).

Also merges the fix/realtime-quote-nil-pointer fix (#104) from fix/realtime-quote-nil-pointer.

New US-only APIs

All methods call CheckRegion(path, "US") at entry and return *http.RegionRestrictedError for non-US tokens.

P0 — Quote / Trade / Asset

Method	Path
QuoteContext.CryptoOverview	GET /v1/gemini/crypto-overview
TradeContext.QueryUSOrders	POST /v1/orders/query
TradeContext.USOrderDetail	GET /v3/orders/{order_id}
TradeContext.USAssetOverview	GET /v1/us/assets/overview
TradeContext.USRealizedPL	GET /v1/us/assets/pl/realized

P1 — Fundamental

Method	Path
FundamentalContext.CompanyOverview	GET /v1/stock-info/company-overview
FundamentalContext.ValuationOverview	GET /v1/stock-info/valuation-overview
FundamentalContext.FinancialOverview	GET /v1/stock-info/finn-overview
FundamentalContext.FinancialStatementV3	GET /v1/us/quote/financials/statements
FundamentalContext.KeyFinancialMetrics	GET /v1/stock-info/fin-keyfactor
FundamentalContext.AnalystConsensus	GET /v1/stock-info/fin-consensus
FundamentalContext.ETFDividendInfo	GET /v1/stock-info/etf-dividend-info
FundamentalContext.CompanyDividends	GET /v1/stock-info/company-dividends

P2 — Fundamental

Method	Path
FundamentalContext.ETFFiles	GET /v1/stock-info/etf-files

AP-only guards (synced from Rust PR #554)

Methods that are AP-only now return *http.RegionRestrictedError when called with a US token:

• DCAContext: all 9 methods (List, Create, Update, Pause, Resume, Stop, History, Stats, CheckSupport, CalcDate, SetReminder)
• FundamentalContext.Operating
• MarketContext.BrokerHolding, BrokerHoldingDetail, BrokerHoldingDaily
• QuoteContext.Brokers (WebSocket)

Structured error

http.RegionRestrictedError mirrors Rust's HttpClientError::DcRegionRestricted:

type RegionRestrictedError struct {
    Path     string // API path
    Required string // "US" or "AP"
    Current  string // "US" or "AP"
}
// "this API (/v1/orders/query) is only available in the US data center
//  and is not supported for your AP-region account"

http.Client.CheckRegion(path, "US"|"AP") is the single call site — returns nil on match, *RegionRestrictedError on mismatch.

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