Add C# (.NET) binding over the C ABI hub (#226)

The first language stecker on the C ABI hub: a .NET binding exposing all 514
indicators as idiomatic `IDisposable` classes, generated from `wickra.h`.

## What's here

- **`bindings/csharp/`** — the `Wickra` .NET 8 package. `[LibraryImport]`
  source-generated P/Invoke (`NativeMethods.g.cs`) plus idiomatic wrappers
  (`Indicators.g.cs`), both generated from the committed `bindings/c/include/wickra.h`.
  The binding owns no indicator maths — it only marshals types across the C ABI.
- **Marshalling, verified end-to-end against the native library.** Opaque handles
  cross as `nint` kept alive per call via a `SafeHandle`; `bool` as
  `[MarshalAs(U1)]` (Rust `bool` is one byte); a self-correcting
  `DllImportResolver` validates the loaded library actually exports the Wickra
  ABI. Tests cover one representative per FFI archetype (scalar, candle, pairwise,
  multi-output, bars, profile, values-profile, order-book / array-input) plus
  exact Sma reference values.
- **NuGet packaging** — `dotnet pack` produces `Wickra.<version>.nupkg`; the
  release pipeline stages prebuilt native libraries under `runtimes/<rid>/native/`
  for six target triples (win/linux/osx × x64/arm64).
- **`examples/csharp/`** — nine examples mirroring `examples/c/`: streaming,
  backtest, multi_timeframe, parallel_assets, three strategies, and
  fetch_btcusdt + live_binance.
- **CI** — a `csharp` job on the three OSes builds the C ABI, tests the binding,
  and runs the offline examples. **Release** — a gated `csharp-publish` job packs
  and pushes to NuGet (gated on `NUGET_API_KEY`, independent of the GitHub-release
  job so a C# hiccup never blocks the C/C++ asset release).
- **Docs consistency wave** — README, CONTRIBUTING, CHANGELOG, examples/README,
  the issue / PR templates, `sync-about.yml`, and `.gitattributes`.

The native Python / Node / WASM bindings and the C ABI are untouched; this is
additive. Publishing to NuGet stays gated behind the release tag and the secret.

examples/README.md

@@ -48,6 +48,27 @@ three `strategy_*`) build against the bundled datasets and run under `ctest`.
 run in CI; run them by hand. `parallel_assets` links OpenMP when the toolchain
 provides it and falls back to a single-threaded run otherwise.
 
+## C# / .NET — `examples/csharp/`
+
+Build the C ABI library first (`cargo build -p wickra-c --release`), then run any
+example with the .NET 8 SDK; the binding resolves the native library automatically.
+
+| Example | What it does | Run |
+| --- | --- | --- |
+| `streaming` | Feed a synthetic price series through SMA / EMA / RSI / MACD tick by tick. | `dotnet run --project examples/csharp/streaming` |
+| `backtest` | Basket of indicators over an OHLCV series (CSV arg or synthetic). | `dotnet run --project examples/csharp/backtest -- <ohlcv.csv>` |
+| `multi_timeframe` | Resample a 1-minute series to 5m / 15m and print an indicator per timeframe. | `dotnet run --project examples/csharp/multi_timeframe` |
+| `parallel_assets` | SMA(20) batch over a panel, serial vs `Parallel.For`, with speedup. | `dotnet run -c Release --project examples/csharp/parallel_assets` |
+| `strategy_rsi_mean_reversion` | RSI(14) mean-reversion with PnL / Sharpe / max-DD summary. | `dotnet run -c Release --project examples/csharp/strategy_rsi_mean_reversion` |
+| `strategy_macd_adx` | Trend-follower: MACD crossover entries gated by ADX(14) > 20. | `dotnet run -c Release --project examples/csharp/strategy_macd_adx` |
+| `strategy_bollinger_squeeze` | Bollinger-squeeze breakout with an ATR(14) trailing stop. | `dotnet run -c Release --project examples/csharp/strategy_bollinger_squeeze` |
+| `fetch_btcusdt` | Download real BTCUSDT klines from the Binance REST API into a CSV. | `dotnet run --project examples/csharp/fetch_btcusdt` |
+| `live_binance` | Stream live Binance klines through EMA(20) over a WebSocket. | `dotnet run --project examples/csharp/live_binance` |
+
+The offline examples run on deterministic synthetic data (and under CI on all
+three OSes); `fetch_btcusdt` and `live_binance` reach the network and are built
+but not run in CI.
+
 ## Python — `examples/python/`
 
 | Example | What it does | Run |