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.

CHANGELOG.md

@@ -6,6 +6,13 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
+### Changed
+- Package registry metadata for better discoverability:
+  - R (R-universe): added the R-universe URL and `X-schema.org-keywords` to the
+    R `DESCRIPTION`, plus a package logo at `bindings/r/man/figures/logo.png`
+    (pkgdown convention).
+  - Python (PyPI): added a `Documentation` project URL.
+  - C# (NuGet): added a package icon via `PackageIcon`.
 
 ## [0.8.5] - 2026-06-11
 ### Fixed