examples: move the bundled BTCUSDT datasets to a top-level examples/data/

The seven BTCUSDT OHLCV datasets used to live under
crates/wickra/examples/data/, which buried them inside a Rust crate even
though the Node backtest example and the upcoming Rust/Node/WASM example
restructure need to reach them too. Move them to the workspace-level
examples/data/ so every language's examples can resolve the same path.

The bench (crates/wickra/benches/indicators.rs), the example_data
integration test, fetch_btcusdt.rs and the Node backtest example all take
the new ../../examples/data/ path; Data-Layer.md, examples/README.md and
the CHANGELOG entry are updated to match. No data file content changes.

examples/data/btcusdt-1month.csv

@@ -0,0 +1,106 @@
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