feat(data): native Binance REST kline fetcher in 9 languages (#315)

Adds `BinanceRest::fetch_klines` to `wickra-data`: a blocking historical kline
downloader (`GET /api/v3/klines`) and the historical counterpart to the F4 live
`BinanceFeed`. It is the last native data-layer primitive needed to drop
third-party HTTP/JSON download helpers (`jackson`, `jsonlite`, `urllib`, …) from
the examples.

## What

- **Core** (`wickra-data`): `fetch_klines(symbol, interval, limit, start?, end?)`
  built on `ureq` with native-tls — sharing the exact same TLS backend
  (native-tls 0.2 / SChannel) as the existing tokio-tungstenite live feed, so the
  two pull one TLS stack, not two. Parses Binance's 12-element array rows via the
  existing serde infrastructure into validated `Candle`s. Blocking by design (a
  one-shot request needs no async runtime; the FFI boundary is synchronous
  anyway). Nine mock-HTTP-server tests cover parse / empty / limit / transport /
  JSON / invariant-violation paths.
- **C ABI**: `wickra_binance_fetch_klines(...)` (blocking drain into a caller
  buffer, `-1` on error) + regenerated cbindgen header and its vendored Go copy.
- **Bindings**: native Node `fetchBinanceKlines` / Python `fetch_binance_klines`;
  generated Go `FetchBinanceKlines` / C# `BinanceFeed.FetchKlines` / Java
  `BinanceFeed.fetchKlines` / R `fetch_binance_klines`. C / C++ call the C ABI
  directly. **WASM is excluded** (browsers use the host `fetch`).

The four C-ABI bindings are regenerated from the ScriptHelpers generators (not
hand-edited); the regen diff is exactly the new wrapper in each.

## Verification

All ten toolchains green locally: Rust (`cargo test`/`clippy`/`fmt`), Node, Python,
Go, C#, Java, R (`R CMD INSTALL` + smoke), WASM (`cargo check`, confirmed `ureq`
is not pulled). Each binding has an error-path smoke test; the parse/HTTP success
path is covered by the Rust mock-server tests.

No release in this PR — ships with the data-layer + numpy bundle later.

bindings/r/R/indicators.R

@@ -4180,3 +4180,24 @@ binance_close <- function(feed) {
   invisible(NULL)
 }
 
+#' Fetch historical Binance klines over REST
+#'
+#' Downloads up to `limit` (`1:1000`) historical klines for `symbol` at the given
+#' `interval` code (an integer `0:15`, the same order as the other bindings).
+#' `start_ms`/`end_ms` are optional inclusive Unix-millisecond bounds (a negative
+#' value means unset); `base_url` overrides the host (`NULL` = production). Returns
+#' an `n x 6` numeric matrix with columns `open`, `high`, `low`, `close`,
+#' `volume`, `timestamp`. Blocks until the response arrives. Not available in the
+#' wasm (r-universe/webR) build, which has no raw sockets.
+#'
+#' @keywords internal
+#' @export
+fetch_binance_klines <- function(symbol, interval, limit, start_ms = -1,
+                                 end_ms = -1, base_url = NULL) {
+  m <- .Call("wk_binance_fetch_klines", symbol, as.integer(interval),
+             as.integer(limit), as.numeric(start_ms), as.numeric(end_ms),
+             base_url, PACKAGE = "wickra")
+  colnames(m) <- c("open", "high", "low", "close", "volume", "timestamp")
+  m
+}
+