Function dispatch 

pub fn dispatch(
    mode: DispatchMode,
    canonical_uri: &str,
    env: &dyn DeepLinkEnv,
) -> Result<DispatchOutcome, String>

Decide-and-dispatch. Pure decision logic over the DeepLinkEnv seam:

• DispatchMode::Server → DispatchOutcome::ServeBrowser (no upsell); the handler is never probed and no URL is opened.
• DispatchMode::Auto → if the handler is registered, build the deep link, open it, and return DispatchOutcome::HandedOff; otherwise DispatchOutcome::ServeBrowser with the upsell.
• DispatchMode::Desktop → if the handler is registered, hand off the same way; otherwise DispatchOutcome::DesktopNotInstalled.

canonical_uri must already be canonicalised (see canonicalize_target_uri); it is embedded verbatim (percent-encoded) in the deep link and never carries a credential.