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.

rust/core/src/models.rs

@@ -1,5 +1,5 @@
-//! Data model shared by storage, the session manager and the UI. Field names serialise exactly as
-//! the Go structs did, so database JSON columns and `.link`/`.nexus` exports round-trip unchanged.
+//! Data model shared by storage, the session manager and the UI. Field names are the JSON keys
+//! used in the database's config columns and in `.nexus` exports.
 
 use chrono::{DateTime, FixedOffset, Local};
 use serde::{Deserialize, Deserializer, Serialize};