Correct stale statements on tala.run and rename the vxs storage keys

An audit of the site against the compiler repository found statements
that no longer hold, in all four languages:

- the overview named BUILD.kts as the project definition, while Tazel
  reads one of five names; it now says a project file, BUILD.kts or one
  of four other names;
- the overview and the getting-started guide named Windows and macOS as
  the only development hosts. Ubuntu 26.04 LTS and Fedora 43 are official
  hosts too, and the guide says which toolchain they use;
- the getting-started guide listed the counterparts of the examples in
  C#, C++ and Java; the examples also have Rust ones;
- the project-file page wrote build/debug/Example without the .exe a
  Windows build has.

The talac, Tazel and syntax pages were checked against Documents/CLI.md,
Documents/PROJECT_FILES.md and Spec/Language and are otherwise accurate.
Of the 43 links in the repository, only the address inside the Fira Sans
licence text no longer answers; that text is third-party and verbatim.

The browser storage keys still carried the former name of the language:
vxs-locale-choice, vxs-theme and vxs-forum-theme become tala-locale-choice,
tala-theme and tala-forum-theme. A value under a former key moves to the
current one on the first page load, so no visitor loses a choice; a forum
test covers the move. The npm package is renamed from
@visual-xsharp/website to @progmasoft/tala-website, and the forum assets
get a new version query so Cloudflare serves the new script.

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