Add HelpTopic inventory type for non-command help entries

[gfnord]

Local branch for testing the HelpTopic registry on top of all existing fork changes (hostname cloaking, LIST command, etc.).

Mirrors upstream PR Libera-Chat#189.

What this adds

A compile-time registry for free-form help topics not tied to any command handler — channel modes, user modes, or any concept a user might query with HELP <topic>.

Register a topic anywhere with inventory::submit!:

inventory::submit!(HelpTopic {
    topic: "CMODE_SECRET",
    lines: &[
        "CMODE_SECRET (+s)",
        "",
        "Marks the channel as secret. Secret channels are hidden from /LIST",
        "and /WHOIS for users who are not members.",
    ],
});

New API on CommandDispatcher (and forwarded on ClientServer):

• get_help_topic(topic) — case-insensitive lookup
• iter_help_topics() — enumerate all topics

How a HELP handler should use both registries

// command docs (from #[command_handler] doc comments, see Libera-Chat/sable#129)
let lines = server.get_command_docs(&topic)
    .or_else(|| server.get_help_topic(&topic));  // this PR

match lines {
    Some(lines) => { /* send 704/705/706 numerics */ }
    None        => { /* send 524 HelpNotFound */ }
}