Add the R binding over the C ABI hub (#230)

Adds an R binding (`bindings/r`) over the C ABI hub — the third language stecker after C# and Go, reaching the hub through R's native `.Call` interface (not extendr).

## What's here
- **`bindings/r`** — an R package exposing all 514 indicators as constructors that return a `wickra_indicator` object with generic `update`/`batch`/`reset` methods. The C glue (`src/wickra.c`) and R wrappers (`R/indicators.R`) are generated from `bindings/c/include/wickra.h` (same archetype taxonomy as the C#/Go generators: scalar/batch, multi-output, bars, profile, profile-values, array-input). The opaque handle is an R external pointer freed by a registered finalizer; multi-output returns a named vector (`NA` at warmup), bars a matrix, profiles a list.
- **`examples/r`** — the full example suite mirroring C/C#/Go: streaming, backtest, multi_timeframe, parallel_assets (`mclapply`), three strategies, and `fetch_btcusdt`/`live_binance`.
- **CI** — an `r` job builds the C ABI library, installs the package, runs the `testthat` suite and the offline examples on Linux, macOS and Windows (`R CMD check` is clean: 0 warnings, 0 notes).
- **Docs** — R added to the README languages table, project layout, building/testing, CONTRIBUTING binding table + regenerate note, ARCHITECTURE, examples index, issue/PR templates, the About-description template, and the other binding READMEs.

## Linking / distribution
The package compiles a thin `.Call` glue layer against the prebuilt C ABI library (header via `WICKRA_INCLUDE_DIR`, library via `WICKRA_LIB_DIR`). On Windows the package's own `wickra.dll` would collide with the C ABI's `wickra.dll`, so `configure.win` stages a renamed copy (`wickra_abi.dll`) and builds an import library referencing it; `install.libs.R` bundles the DLL and `.onLoad` puts it on the load path. On Linux/macOS the rpath locates the shared library. No `release.yml` change — R is distributed via r-universe / source install (gated).

No Rust crate or `Cargo.toml` change — the R package is standalone and additive.

CONTRIBUTING.md

@@ -24,6 +24,7 @@ licensed as above, without any additional terms or conditions.
 | `bindings/c` | C ABI — `cdylib` + `staticlib` + generated `include/wickra.h`. The hub for C / C++ and any C-capable language. |
 | `bindings/csharp` | .NET binding over the C ABI (`Wickra` on NuGet) — `[LibraryImport]` P/Invoke generated from `wickra.h`. |
 | `bindings/go` | Go binding over the C ABI via cgo (module tag `bindings/go/vX.Y.Z`) — wrappers generated from `wickra.h`. |
+| `bindings/r` | R binding over the C ABI via `.Call` (R package) — C glue + R wrappers generated from `wickra.h`. |
 | `examples/` | Runnable examples. |
 | `docs/` | Pointer to the documentation site (docs.wickra.org); the docs live in the `wickra-lib/wickra-docs` repo. |
 
@@ -110,7 +111,9 @@ installed. Dependabot also keeps the `.github/requirements` pins current.
   commit `src/lib.rs` + `include/wickra.h`. The C# binding (`bindings/csharp`) is
   generated from `wickra.h`, so regenerate and commit its `Generated/*.g.cs` too.
   The Go binding (`bindings/go`) is likewise generated from `wickra.h`, so
-  regenerate and commit `indicators_gen.go` (`gofmt`-clean).
+  regenerate and commit `indicators_gen.go` (`gofmt`-clean). The R binding
+  (`bindings/r`) is generated from `wickra.h` too, so regenerate and commit
+  `src/wickra.c` + `R/indicators.R`.
 - **Docs.** Update the relevant page on the
   [documentation site](https://docs.wickra.org) and the
   `README.md` when behaviour or the public API changes. The docs live in