feat(fund): add FundContext for the mutual-fund channel

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Changed
+
+- **Breaking:** `trade.FundPosition` — the fund identifier field is renamed from `Symbol` (which carried the ISIN) to `CounterID`, matching the Rust core and the fund channel. It now holds the full fund `counter_id` (e.g. `UT/FD/HK0000384492`); the ISIN is the last `/`-separated segment. The wire type reads the new `counter_id` key and still accepts the legacy `symbol` key during the transition.
+
 ## [v0.28.0] - 2026-09-28
 
 ### Breaking changes
@@ -20,6 +24,12 @@
 
 ### Added
 
+- **Fund (mutual fund)** — new `fund.FundContext` for the fund channel (ports longbridge/openapi), mirroring the Rust core field-for-field across 28 methods:
+  - Catalog / market data: `HotFunds` (`GET /v1/fund/hot-funds`), `Funds` (`POST /v1/fund/funds`), `Filters` (`GET /v1/fund/filters`), `Detail` (`GET /v1/fund/funds/detail`), `Analysis` / `AnalysisDetail` / `Trend` / `PerformanceComparison` (`.../analysis`, `.../analysis/detail`, `.../trend`, `.../performance/comparison`), `AnnualReturns` / `QuarterlyReturns` (`.../returns/annual`, `.../returns/quarterly`), `Performance` (`.../performance`), `Nav` / `NavHistory` / `NavRange` (`.../nav`, `.../nav-history`, `.../nav-range`), `Holdings` (`.../holdings`) and `StockHoldings` (`.../stock-holdings`, reverse lookup)
+  - User positions: `Positions` (`GET /v1/asset/funds`), `Position` (`.../detail`), `PositionPerformance` / `PositionProfits` / `PositionNav` / `PositionDividends` (`.../performance`, `/profits`, `/nav-history`, `/dividends`)
+  - Orders & trading: `Orders` (`GET /v1/fund/orders`), `Order` (`GET /v1/fund/orders/{id}`), `Transactions` (`GET /v1/fund/transactions`), `ValidateOrder` (`POST /v1/fund/orders/validate`), `SubmitOrder` (`POST /v1/fund/orders`) and `CancelOrder` (`POST /v1/fund/orders/{id}/cancel`)
+  - The fund channel is the deliberate **exception** to the release-wide symbol migration above: funds are identified by `counter_id` (e.g. `UT/FD/HK0000384492`), which cannot be converted to a user symbol. Because it contains `/` it cannot sit in the URL path, so single-fund endpoints take it as a `counter_id` query parameter (fixed sub-paths like `/v1/fund/funds/detail`) rather than a path segment; the order request filter carries repeated `counter_id` query params. Three batch-backed endpoints — `Performance`, `Nav` and `PositionPerformance` — instead send the identifier as a one-element JSON array in a `counter_ids` query parameter, matching their backend contract
+  - Numeric `int64` fields use the string-tolerant `jsontypes.Int64` type (the backend may send them as a JSON number or a quoted string; empty / `null` become `0`); unix-second timestamps are `int64`; nested response objects the backend may return as `null` (`FundDetail.AssetAllocation`, `FundTrend.ContrastPerformances`, `FundPositionDetail.DetailValues`, `FundOrderDetail.Order`) are modeled as pointers so those responses deserialize instead of erroring on `null`; server-defined "any" structures (fund filters, analysis ability/cost blocks, trend / comparison performance series) are preserved as `json.RawMessage`
 - **Grid trading** — new `grid.GridContext` for grid-order management: `Submit` / `Replace` / `Cancel` / `Suspend` / `Restart` grid orders, `List` (paged) and `ListByIds`, `Detail` and `TriggerHistory`, `SubmitStrategyQuestionnaire` (strategy risk-disclosure), and `SymbolInfo` (returns `GridSymbolInfo`: name, last price, lot sizes, price-step rules, channel/authorization) — the security info needed to build a grid order
 - **Multi-leg option orders** (ports longbridge/openapi #575, #589, #590) — new `TradeContext.SubmitMultiLeg` (`POST /v1/trade/order/multileg`) submits a multi-leg option combination order (vertical spreads, straddles, strangles, collars, calendar spreads, etc.) whose legs are placed together as a single strategy order. Takes `Side`, `OrderType`, `SubmittedQuantity`, `Strategy` (`MultiLegStrategy`), a list of `SubmitMultiLegOrderLeg` (`Symbol` + `RatioQuantity`), and optional `SubmittedPrice` / `Remark` / `ClientRequestId`; returns the order ID like `SubmitOrder`. `Order`, `OrderDetail`, and the `PushOrderChanged` order-changed push gain an optional `MultiLeg *MultiLegInfo` field — present only for multi-leg orders — carrying `Strategy`, `StrategyName`, `MultilegId`, `Code`, and the combination `Legs` (each with `Symbol`, `Side`, `Position`, `RatioQuantity`, `StrikePrice`, `ExpireDate`, `ContractDirection`). New enums `MultiLegStrategy` (`CoveredCall` … `Strangle`, plus `CalendarCallSpread` / `CalendarPutSpread`), `MultiLegPosition` (`LONG` / `SHORT`), and `ContractDirection` (`C` / `P`)