test: replace the Go compatibility fixtures with native round-trip tests

The Go-generated link.db, profiles.link and rec1.cast fixtures and the
tests that read them are removed. Their coverage is kept by tests on
data this build writes: a fresh database round-trips SSH profiles (env
vars, login scripts, forwards), serial profiles, IRC room state, known
hosts, snippets and preferences, and encrypted exports round-trip and
reject a wrong password. The import dialog no longer offers .link files
and the docs stop mentioning them.

docs/overview.md

@@ -41,7 +41,7 @@ Use Nexus Terminal when you need to:
 - Multi-select profiles with Ctrl/Cmd-click for bulk connect, disconnect, tagging, moving between groups and deleting.
 - Test basic host and port reachability before connecting.
 - Import existing OpenSSH config entries from `~/.ssh/config`, including hostnames, usernames, ports, identity files and jump hosts.
-- Export and import profiles using encrypted `.nexus` bundles; older `.link` bundles from Alchementary Link import too.
+- Export and import profiles using encrypted `.nexus` bundles.
 - Plain JSON export and import, with secrets redacted unless you choose to include them.
 - Choose import behaviour for duplicates: skip, overwrite or duplicate.