feat(clients): add Go, Rust, C# and Dart clients for ResolveSpec and FunctionSpec

clients/resolvespec-go/README.md

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+# resolvespec-go
+
+Go client for ResolveSpec (JSON body) and FunctionSpec. Module: `github.com/bitechdev/ResolveSpec/clients/resolvespec-go`. Stdlib only.
+
+## Clients
+
+| Type | Constructor | Methods |
+|---|---|---|
+| `Client` | `NewClient(baseURL, opts...)` | `GetMetadata` `Read` `Create` `Update` `Delete` |
+| `FuncSpecClient` | `NewFuncSpecClient(baseURL, opts...)` | `Query` `QueryList` `Do` |
+
+Client options: `WithToken`, `WithHeader`, `WithHTTPClient`. Precedence: Content-Type < custom headers < bearer token.
+
+## ResolveSpec
+
+- All methods take `ctx`; `Read`/`Update`/`Delete` take `RecordID` (`nil`, int/string → URL, `[]string` → body).
+- `Options` fields use pointers for optional ints/bools (`Int(n)`, `Bool(b)`).
+- Result: `*Response{Success, Data (raw JSON), Metadata}`; `resp.Decode(&v)`.
+
+## FunctionSpec
+
+- Routes are server-defined: pass the `path`.
+- `Params` → query string (slice → repeated keys, bool → `true`/`false`).
+- `FuncSpecOptions` → `X-*` headers: `Filters`, `SearchFilters`, `CustomSQLWhere`, `CustomSQLOr`, `Sort`, `Limit`, `Offset`, `Distinct`, `SkipCount`, `SkipCache`, `ResponseFormat`.
+- `QueryList` fills `Metadata` from `Content-Range` (`items a-b/total`); 206 is success.
+
+## Server quirks
+
+- `Sort` is raw SQL in ORDER BY (client sends `col ASC|DESC`).
+- One search operator per column.
+- Values starting `ZIP_` / `__` are base64-decoded by the server.
+- Non-ASCII, control chars and edge spaces are auto-encoded (`ZIP_`).
+
+## Errors
+
+`*Error{StatusCode, APIError{Code, Message, Detail, SQL}}`.
+
+## Test
+
+`go test ./...`