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.

ARCHITECTURE.md

@@ -27,21 +27,26 @@ or replace lives behind a separate crate boundary.
    │   no I/O, no deps    │            │  optional features │
    └──────────────────────┘            └────────────────────┘
                 ▲
-                │ (every binding wraps the same core)
-                │
-   ┌───────────┬────────────┴┬──────────────┬──────────────────────┐
-   │           │             │              │                      │
-┌──▼─────┐ ┌───▼────┐ ┌──────▼──────┐ ┌─────▼──────────────┐
-│ Python │ │  Node  │ │    WASM     │ │  C ABI (cbindgen)  │
-│ (PyO3) │ │(napi-rs)│ │(wasm-bindgen)│ │  cdylib + header   │
-└────────┘ └────────┘ └─────────────┘ └────────────────────┘
+                │  every binding wraps the same core
+   ┌────────────┼────────────┬────────────────┐
+   │            │            │                │
+┌──▼───────┐ ┌──▼───────┐ ┌──▼───────────┐ ┌──▼──────────────────┐
+│  Python  │ │   Node   │ │     WASM     │ │  C ABI (cbindgen)   │
+│  (PyO3)  │ │ (napi-rs)│ │(wasm-bindgen)│ │   cdylib + header   │
+└──────────┘ └──────────┘ └──────────────┘ └─────────┬───────────┘
+                                                     │  linked by
+                                          ┌──────────▼──────────┐
+                                          │  C · C++ · C# · Go  │
+                                          │     · Java · R      │
+                                          └─────────────────────┘
 ```
 
 Python, Node and WASM are *native* Rust bindings (PyO3 / napi-rs /
 wasm-bindgen). The C ABI is the *hub* every other C-capable language links
 against: it builds to a `cdylib`/`staticlib` plus a generated `wickra.h`, and
-C / C++ / Go / C# / Java / R consume that one artifact rather than each
-re-wrapping the core.
+downstream languages link that one artifact rather than each re-wrapping the
+core. C and C++ link it directly; the **C# / .NET** binding (`bindings/csharp`,
+on NuGet) is generated from `wickra.h`, with Go / Java / R planned the same way.
 
 | Crate | Path | What it owns | Public deps |
 |---|---|---|---|