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.

bindings/csharp/README.md

@@ -0,0 +1,78 @@
+# Wickra — .NET
+
+[![CI](https://github.com/wickra-lib/wickra/actions/workflows/ci.yml/badge.svg)](https://github.com/wickra-lib/wickra/actions/workflows/ci.yml)
+[![codecov](https://codecov.io/gh/wickra-lib/wickra/branch/main/graph/badge.svg)](https://codecov.io/gh/wickra-lib/wickra)
+[![NuGet](https://img.shields.io/nuget/v/Wickra.svg?logo=nuget&color=blue)](https://www.nuget.org/packages/Wickra)
+[![License: MIT OR Apache-2.0](https://img.shields.io/badge/license-MIT_OR_Apache--2.0-blue)](https://github.com/wickra-lib/wickra#license)
+
+**Streaming-first technical indicators for .NET. `dotnet add package Wickra` —
+prebuilt native library, no system dependencies.**
+
+Wickra is a multi-language technical-analysis library with a Rust core and
+bindings for Python, Node.js and WebAssembly, plus a C ABI for C/C++, C# and any
+other C-capable language. Every indicator is an O(1)
+streaming state machine, so live trading bots and historical backtests share
+the exact same implementation. This package is the .NET binding; it consumes the
+C ABI hub through `[LibraryImport]` P/Invoke and exposes all 514 streaming-first
+indicators as idiomatic `IDisposable` classes.
+
+## Install
+
+```bash
+dotnet add package Wickra
+```
+
+The native library ships prebuilt per platform (Linux, macOS, Windows — x64 and
+arm64) under `runtimes/<rid>/native/`, selected automatically. There is nothing
+to compile. Targets .NET 8 and later.
+
+## Quick start
+
+```csharp
+using Wickra;
+
+// Batch: run an indicator over a whole series (NaN at warmup positions).
+var prices = Enumerable.Range(0, 1000).Select(i => 100.0 + i * 0.1).ToArray();
+using var sma = new Sma(20);
+double[] values = sma.Batch(prices);
+
+// Streaming: the same indicator, fed tick by tick in O(1).
+using var rsi = new Rsi(14);
+foreach (var price in liveFeed)
+{
+    var value = rsi.Update(price); // NaN during warmup, no recomputation
+    if (double.IsFinite(value) && value > 70)
+    {
+        Console.WriteLine("overbought");
+    }
+}
+```
+
+`Batch(prices)` and feeding the same prices through `Update()` produce identical
+values — the equivalence is enforced by the test suite. Multi-output indicators
+(MACD, Bollinger, ADX, …) return a nullable `record struct`, `null` while warming up.
+
+## Documentation
+
+The full indicator catalogue, guides, quickstarts, and API reference live in
+the main repository and documentation site:
+
+- **Repository & full indicator list:** <https://github.com/wickra-lib/wickra>
+- **Docs** (quickstarts, cookbook, TA-Lib migration): <https://docs.wickra.org>
+- **Runnable examples:** [`examples/csharp/`](https://github.com/wickra-lib/wickra/tree/main/examples/csharp)
+
+Wickra ships native bindings for Python, Node.js, WebAssembly and Rust, plus a
+C ABI hub that any C-capable language (C, C++, Go, C#, Java, R) links against —
+all exposing the same indicators from the shared, `unsafe`-forbidden Rust core.
+
+## Disclaimer
+
+Wickra is an indicator toolkit, not a trading system. The values it computes
+are deterministic transforms of the input data — they are not financial advice
+and do not predict the market. Any use in a live trading context is at your own
+risk. The library is provided **as is**, without warranty of any kind.
+
+## License
+
+Licensed under either of [Apache-2.0](https://github.com/wickra-lib/wickra/blob/main/LICENSE-APACHE)
+or [MIT](https://github.com/wickra-lib/wickra/blob/main/LICENSE-MIT) at your option.