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/Wickra.Tests/ArchetypeTests.cs

@@ -0,0 +1,144 @@
+using Wickra;
+using Xunit;
+
+namespace Wickra.Tests;
+
+/// <summary>
+/// One representative per FFI archetype, exercising every marshalling path the
+/// generator produces (scalar, candle, pairwise, multi-output, bars, profile,
+/// values-profile, array-input). Garbage marshalling surfaces as NaN, wild
+/// values, or crashes — so finite/sane assertions are the real check.
+/// </summary>
+public class ArchetypeTests
+{
+    private static (double open, double high, double low, double close, double volume, long ts) Candle(int i)
+    {
+        var close = 100.0 + 10.0 * Math.Sin(i * 0.3);
+        var open = 100.0 + 10.0 * Math.Sin((i - 1) * 0.3);
+        var high = Math.Max(open, close) + 1.0;
+        var low = Math.Min(open, close) - 1.0;
+        return (open, high, low, close, 1_000.0, i * 60_000L);
+    }
+
+    [Fact]
+    public void Scalar_Ema_IsFiniteAfterWarmup()
+    {
+        using var ema = new Ema(3);
+        double last = double.NaN;
+        for (var i = 1; i <= 10; i++)
+        {
+            last = ema.Update(i);
+        }
+
+        Assert.True(double.IsFinite(last));
+        Assert.InRange(last, 1.0, 10.0);
+    }
+
+    [Fact]
+    public void Candle_Atr_IsFinitePositive()
+    {
+        using var atr = new Atr(3);
+        double last = double.NaN;
+        for (var i = 0; i < 20; i++)
+        {
+            var (o, h, l, c, v, ts) = Candle(i);
+            last = atr.Update(o, h, l, c, v, ts);
+        }
+
+        Assert.True(double.IsFinite(last));
+        Assert.True(last > 0.0);
+    }
+
+    [Fact]
+    public void Pairwise_Beta_IsFinite()
+    {
+        using var beta = new Beta(5);
+        double last = double.NaN;
+        for (var i = 0; i < 30; i++)
+        {
+            var market = 100.0 + 10.0 * Math.Sin(i * 0.5);
+            var asset = 50.0 + 6.0 * Math.Sin(i * 0.5 + 0.2);
+            last = beta.Update(market, asset);
+        }
+
+        Assert.True(double.IsFinite(last));
+    }
+
+    [Fact]
+    public void MultiOutput_Adx_ReturnsFiniteStruct()
+    {
+        using var adx = new Adx(5);
+        AdxOutput? result = null;
+        for (var i = 0; i < 60; i++)
+        {
+            var (o, h, l, c, v, ts) = Candle(i);
+            result = adx.Update(o, h, l, c, v, ts);
+        }
+
+        Assert.NotNull(result);
+        Assert.True(double.IsFinite(result!.Value.Adx));
+        Assert.True(double.IsFinite(result.Value.PlusDi));
+        Assert.True(double.IsFinite(result.Value.MinusDi));
+    }
+
+    [Fact]
+    public void Bars_DollarBars_EmitsBars()
+    {
+        using var bars = new DollarBars(5_000.0);
+        var total = 0;
+        for (var i = 0; i < 200; i++)
+        {
+            var (o, h, l, c, v, ts) = Candle(i);
+            total += bars.Update(o, h, l, c, v, ts).Length;
+        }
+
+        Assert.True(total > 0);
+    }
+
+    [Fact]
+    public void Profile_VolumeProfile_ReturnsValues()
+    {
+        using var profile = new VolumeProfile(20, 8);
+        VolumeProfileOutputScalars? result = null;
+        for (var i = 0; i < 60; i++)
+        {
+            var (o, h, l, c, v, ts) = Candle(i);
+            result = profile.Update(o, h, l, c, v, ts);
+        }
+
+        Assert.NotNull(result);
+        Assert.NotNull(result!.Value.Values);
+        Assert.True(result.Value.PriceLow <= result.Value.PriceHigh);
+    }
+
+    [Fact]
+    public void ProfileValues_DayOfWeekProfile_NoCrash()
+    {
+        using var profile = new DayOfWeekProfile(0);
+        double[]? result = null;
+        for (var i = 0; i < 60; i++)
+        {
+            var close = 100.0 + 5.0 * Math.Sin(i * 0.2);
+            // one day apart so the day-of-week buckets fill
+            result = profile.Update(close, close + 1, close - 1, close, 1_000.0, i * 86_400_000L);
+        }
+
+        if (result is not null)
+        {
+            Assert.All(result, v => Assert.True(double.IsFinite(v)));
+        }
+    }
+
+    [Fact]
+    public void ArrayInput_DepthSlope_IsFinite()
+    {
+        using var slope = new DepthSlope();
+        ReadOnlySpan<double> bidPrice = stackalloc double[] { 99.0, 98.0, 97.0 };
+        ReadOnlySpan<double> bidSize = stackalloc double[] { 10.0, 20.0, 30.0 };
+        ReadOnlySpan<double> askPrice = stackalloc double[] { 101.0, 102.0, 103.0 };
+        ReadOnlySpan<double> askSize = stackalloc double[] { 12.0, 22.0, 32.0 };
+
+        var result = slope.Update(bidPrice, bidSize, askPrice, askSize);
+        Assert.True(double.IsFinite(result));
+    }
+}

bindings/csharp/Wickra.Tests/SmaTests.cs

@@ -0,0 +1,51 @@
+using Wickra;
+using Xunit;
+
+namespace Wickra.Tests;
+
+public class SmaTests
+{
+    [Fact]
+    public void StreamingMatchesReference()
+    {
+        using var sma = new Sma(3);
+        Assert.True(double.IsNaN(sma.Update(1)));
+        Assert.True(double.IsNaN(sma.Update(2)));
+        Assert.Equal(2.0, sma.Update(3), 9);
+        Assert.Equal(3.0, sma.Update(4), 9);
+        Assert.Equal(4.0, sma.Update(5), 9);
+    }
+
+    [Fact]
+    public void BatchMatchesStreaming()
+    {
+        using var sma = new Sma(3);
+        var output = sma.Batch(new double[] { 1, 2, 3, 4, 5 });
+
+        Assert.True(double.IsNaN(output[0]));
+        Assert.True(double.IsNaN(output[1]));
+        Assert.Equal(2.0, output[2], 9);
+        Assert.Equal(3.0, output[3], 9);
+        Assert.Equal(4.0, output[4], 9);
+    }
+
+    [Fact]
+    public void ResetClearsState()
+    {
+        using var sma = new Sma(3);
+        sma.Update(1);
+        sma.Update(2);
+        sma.Update(3);
+        sma.Reset();
+        Assert.True(double.IsNaN(sma.Update(10)));
+    }
+
+    [Fact]
+    public void ZeroPeriodThrows()
+    {
+        // Zero is rejected by the native constructor (returns NULL) -> ArgumentException;
+        // a negative period is caught earlier by the wrapper guard.
+        Assert.Throws<ArgumentException>(() => new Sma(0));
+        Assert.Throws<ArgumentOutOfRangeException>(() => new Sma(-1));
+    }
+}

bindings/csharp/Wickra.Tests/Wickra.Tests.csproj

@@ -0,0 +1,21 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net8.0</TargetFramework>
+    <LangVersion>latest</LangVersion>
+    <Nullable>enable</Nullable>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <IsPackable>false</IsPackable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.11.1" />
+    <PackageReference Include="xunit" Version="2.9.2" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="2.8.2" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\Wickra\Wickra.csproj" />
+  </ItemGroup>
+
+</Project>