chore: improve package registry metadata (R-universe, PyPI, NuGet) (#259)

Closes the package-page discoverability gaps found while auditing the public registry listings. No code or version change — metadata only; takes effect on the next registry build/publish.

## R — R-universe (`bindings/r`)
- `DESCRIPTION`: add the R-universe URL to `URL:` and a CRAN-permitted `X-schema.org-keywords` field (feeds R-universe's search/ranking).
- Add a package logo at `man/figures/logo.png` (pkgdown convention → shown in the R-universe packages tab) and reference it in the R `README.md`.
- Maintainer email is **unchanged** (`support@wickra.org`).

## Python — PyPI (`bindings/python`)
- `pyproject.toml`: add a `Documentation` project URL (`https://docs.wickra.org`) so it appears in the PyPI sidebar.

## C# — NuGet (`bindings/csharp`)
- Add `icon.png` (brand mark) and `<PackageIcon>` so nuget.org shows the logo instead of the default placeholder.

## Notes
- Rust/crates.io, Node/npm, Go/pkg.go.dev and Java/Maven Central were audited and are already complete for their respective metadata models (no icon/keyword concept on some).
- The repository topic `rstats` was added (replacing the redundant `webassembly`, since `wasm` already covers it) so R is represented alongside the other language tags.

bindings/r/DESCRIPTION

@@ -8,8 +8,12 @@ Description: R bindings for the Wickra technical-analysis library over its C ABI
     the Rust core and the other language bindings, so that live and historical
     evaluation use the exact same implementation.
 License: MIT + file LICENSE | Apache License 2.0
-URL: https://github.com/wickra-lib/wickra, https://docs.wickra.org
+URL: https://github.com/wickra-lib/wickra, https://docs.wickra.org,
+    https://wickra-lib.r-universe.dev
 BugReports: https://github.com/wickra-lib/wickra/issues
+X-schema.org-keywords: technical-analysis, indicators, streaming, trading,
+    finance, quant, algorithmic-trading, backtesting, time-series, ta-lib,
+    candlestick, market-data
 Encoding: UTF-8
 NeedsCompilation: yes
 SystemRequirements: the Wickra C ABI shared library, downloaded automatically at

bindings/r/README.md

@@ -1,4 +1,4 @@
-# Wickra — R
+# Wickra — R <img src="man/figures/logo.png" align="right" height="120" alt="Wickra logo" />
 
 [![CI](https://github.com/wickra-lib/wickra/actions/workflows/ci.yml/badge.svg)](https://github.com/wickra-lib/wickra/actions/workflows/ci.yml)
 [![codecov](https://codecov.io/gh/wickra-lib/wickra/branch/main/graph/badge.svg)](https://codecov.io/gh/wickra-lib/wickra)