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/tests/fixtures/README.md

@@ -1,11 +0,0 @@
-# Compatibility fixtures
-
-Generated by the last Go build of Alchementary Link with `cmd/genfixtures` (see commit f588a79,
-`git show f588a79:cmd/genfixtures/main.go`). Nexus Terminal must keep reading them unchanged:
-
-- `link.db` — a database through migration 006 with every protocol, a known host, a snippet,
-  IRC channel state and preferences.
-- `profiles.link` — an encrypted export with password `export-pass`.
-- `recordings/rec1.cast` — an asciicast v2 recording.
-
-Don't regenerate them; they are the reference for data written by the old app.

rust/core/tests/fixtures/recordings/rec1.cast

@@ -1,3 +0,0 @@
-{"version":2,"width":80,"height":24,"timestamp":1791633486,"title":"Fixture — host","env":{"TERM":"xterm-256color"}}
-[0.000139144,"o","hello \u001b[1mworld\u001b[0m\r\n"]
-[0.000212149,"i","ls\r"]