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.

Cargo.lock

@@ -1974,7 +1974,6 @@ dependencies = [
  "approx",
  "criterion",
  "proptest",
- "serde_json",
  "wickra-core",
  "wickra-data",
 ]
@@ -2003,10 +2002,19 @@ dependencies = [
  "tokio",
  "tokio-tungstenite",
  "url",
- "wickra",
  "wickra-core",
 ]
 
+[[package]]
+name = "wickra-examples"
+version = "0.0.0"
+dependencies = [
+ "serde_json",
+ "tokio",
+ "wickra",
+ "wickra-data",
+]
+
 [[package]]
 name = "wickra-node"
 version = "0.1.4"