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.
 

docs/user-manual.md

@@ -379,7 +379,7 @@ Connections, disconnections, profile changes, credential access, imports and exp
 ## 13. Import and export
 
 - **Encrypted export**: **File → Export profiles…**, the sidebar footer, the command palette, or **Export…** on a profile's right-click menu. Choose the profiles, set a password and save a `.nexus` file. Credentials saved in the keychain or vault are not included.
-- **Encrypted import**: **File → Import profiles…** or the sidebar footer. Choose a `.nexus` file (or a `.link` file from Alchementary Link), enter its password, and choose what happens when a profile already exists: **Skip existing**, **Overwrite existing** or **Create duplicates**.
+- **Encrypted import**: **File → Import profiles…** or the sidebar footer. Choose a `.nexus` file, enter its password, and choose what happens when a profile already exists: **Skip existing**, **Overwrite existing** or **Create duplicates**.
 - **JSON export and import**: **Export Profiles (JSON)** and **Import Profiles (JSON)** in the command palette. JSON export includes every profile; passwords in login-script `send` steps that answer a password prompt are blanked unless you tick **Include secrets**. Proxy passwords and TOTP secrets live in the keychain or vault, never in the profile, so no export carries them.
 - **OpenSSH config**: **File → Import ~/.ssh/config** or **Settings → About → Import ~/.ssh/config** creates profiles from your `Host` blocks, including hostnames, usernames, ports, identity files and jump hosts.