Transport-agnostic host client + cli/mcp --connect <url> resolver

[lxsaah]

Follow-up to #39 / doc 041. Independent, pure-std PR; only useful once a second transport exists (#tcp / #serial), so sequence it after one of those lands.

Lets an operator pick the transport at runtime, mirroring the scheme:// form records already use for links.

Scope

• aimdb-client: AimxConnection::connect_over(dialer: impl Dialer) (transport-agnostic entry point) + connect_url(url); keep connect(path) as connect_over(UdsDialer::new(path)).
• A dial(url) -> Box<dyn Dialer> resolver mapping unix:// / tcp:// / serial:// (+ bare path, + legacy --socket) to the linked-in transport's Dialer; unknown scheme = clean startup error listing the built-ins.
• aimdb-cli: --connect <url> flag (and --socket kept as unix:// shorthand).
• aimdb-mcp: AIMDB_CONNECT env + config; connection pool re-keyed by URL instead of socket path.
• Build-time menu = which transport crates the binary links in (default: uds + tcp + serial).

Verification

• Resolver unit test: unix:// / tcp:// / serial:// / bare path / legacy --socket map to the right Dialer; unknown scheme rejected with a clear error.
• Existing cli/mcp socket usage + mcp pool keep working unchanged.