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.

.github/workflows/ci.yml

@@ -677,6 +677,56 @@ jobs:
           cmake --build examples/c/build --config Release
           ctest --test-dir examples/c/build -C Release --output-on-failure
 
+  csharp:
+    name: C# on ${{ matrix.os }}
+    runs-on: ${{ matrix.os }}
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [ubuntu-latest, macos-latest, windows-latest]
+    steps:
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+        with:
+          persist-credentials: false
+
+      - name: Install Rust toolchain
+        uses: dtolnay/rust-toolchain@29eef336d9b2848a0b548edc03f92a220660cdb8 # stable branch, 2026-03-27
+
+      - name: Cache cargo
+        uses: Swatinem/rust-cache@e18b497796c12c097a38f9edb9d0641fb99eee32 # v2
+        continue-on-error: true # cache is an optimisation; never block on a stuck/slow restore
+        timeout-minutes: 6
+
+      # The binding links against the C ABI hub at runtime; build it first so the
+      # DllImportResolver finds target/release/wickra.{dll,so,dylib}. .NET 8 SDK is
+      # preinstalled on the GitHub runners, so no setup-dotnet step is needed.
+      - name: Build the C ABI library
+        run: cargo build -p wickra-c --release
+
+      - name: .NET info
+        run: dotnet --info
+
+      - name: Test the C# binding
+        run: dotnet test bindings/csharp/Wickra.Tests/Wickra.Tests.csproj -c Release
+
+      - name: Build the C# examples
+        shell: bash
+        run: |
+          for d in streaming backtest multi_timeframe parallel_assets \
+                   strategy_rsi_mean_reversion strategy_macd_adx strategy_bollinger_squeeze \
+                   fetch_btcusdt live_binance; do
+            dotnet build "examples/csharp/$d" -c Release
+          done
+
+      # Run only the offline examples (fetch_btcusdt / live_binance need network).
+      - name: Run the offline C# examples
+        shell: bash
+        run: |
+          for d in streaming backtest multi_timeframe parallel_assets \
+                   strategy_rsi_mean_reversion strategy_macd_adx strategy_bollinger_squeeze; do
+            dotnet run --project "examples/csharp/$d" -c Release --no-build
+          done
+
   # The cross-library benchmark has moved to a dedicated scheduled workflow
   # (.github/workflows/bench.yml) — see audit finding R10. It runs nightly
   # at 03:00 UTC and on-demand via `workflow_dispatch`, and is no longer on