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/csharp/multi_timeframe/Program.cs

@@ -0,0 +1,44 @@
+using Wickra;
+using Wickra.Examples;
+
+// Resample a 1-minute series into higher timeframes and run an indicator per timeframe.
+var oneMinute = MarketData.SyntheticCandles(1200, startTimestamp: 0, stepMs: 60_000);
+
+Console.WriteLine("EMA(20) of close across timeframes (resampled from 1-minute bars):");
+foreach (var factor in new[] { 1, 5, 15 })
+{
+    var bars = Resample(oneMinute, factor);
+    using var ema = new Ema(20);
+    double last = 0;
+    foreach (var b in bars)
+    {
+        last = ema.Update(b.Close);
+    }
+
+    Console.WriteLine($"  {factor,2}m: {bars.Length,5} bars  EMA(20) last = {last:F4}");
+}
+
+static Bar[] Resample(Bar[] source, int factor)
+{
+    if (factor <= 1)
+    {
+        return source;
+    }
+
+    var output = new List<Bar>();
+    for (var i = 0; i < source.Length; i += factor)
+    {
+        var end = Math.Min(i + factor, source.Length);
+        double high = double.MinValue, low = double.MaxValue, volume = 0;
+        for (var j = i; j < end; j++)
+        {
+            high = Math.Max(high, source[j].High);
+            low = Math.Min(low, source[j].Low);
+            volume += source[j].Volume;
+        }
+
+        output.Add(new Bar(source[i].Open, high, low, source[end - 1].Close, volume, source[i].Timestamp));
+    }
+
+    return output.ToArray();
+}