examples: move Rust examples into a top-level examples/rust/ crate

The three Rust examples (backtest, fetch_btcusdt, live_binance) used to
live each in their own crate's examples/ dir, splitting the example set
across crates and burying it inside the source tree. Move them into a new
workspace member crate at `examples/rust/` (package `wickra-examples`,
`publish = false`) so all language examples sit under one top-level
`examples/<lang>/` tree.

* `examples/rust/Cargo.toml` declares the per-binary deps (wickra,
  wickra-data with the `live-binance` feature always on, serde_json, tokio
  for the macro and current-thread runtime).
* `examples/rust/src/bin/{backtest,fetch_btcusdt,live_binance}.rs` are the
  three migrated binaries; their doc-comments and the fetch_btcusdt output
  path are updated for the new location and run command
  (`cargo run -p wickra-examples --bin <name>`).
* Workspace `Cargo.toml` lists the new member; the now-empty
  `[dev-dependencies]` extras (`wickra`, `tokio` in wickra-data and
  `serde_json` in wickra) that existed only for these examples are dropped.
* The `[[example]] live_binance` table is removed from wickra-data's
  manifest since the file moved out.
* README "Languages" + project-layout, examples/README.md, Quickstart-Rust
  and Data-Layer are pointed at the new paths and commands.

`cargo build -p wickra-examples` and `cargo run --release -p wickra-examples
--bin backtest -- examples/data/btcusdt-1d.csv` both succeed; the rest of
the workspace (core, data, wickra) builds, clippies (`--all-targets -D
warnings`) and tests (508 core + 28 data + 1 integration + 74+3+1
doctests) all stay green.

docs/wiki/Data-Layer.md

@@ -150,10 +150,10 @@ dropped connection, skips non-kline frames (subscription acks, heartbeats),
 applies a read timeout and message-size limits, and tracks a closed flag so a
 deliberately closed stream is not reused.
 
-A runnable example lives at `crates/wickra-data/examples/live_binance.rs`:
+A runnable example lives at `examples/rust/src/bin/live_binance.rs`:
 
 ```bash
-cargo run -p wickra-data --example live_binance --features live-binance
+cargo run -p wickra-examples --bin live_binance
 ```
 
 ## Example datasets
@@ -183,7 +183,7 @@ Binance REST API and needs the system `curl` (shipped with Windows 10+, macOS
 and Linux):
 
 ```bash
-cargo run -p wickra --example fetch_btcusdt
+cargo run -p wickra-examples --bin fetch_btcusdt
 ```
 
 ## See also