Alt-Svc HTTP/3 discovery is dead: set_h3_support early-returns before the Alt-Svc upgrade

[barjin]

🤖 Found by Claude ultrareview — automated high-effort code review. Please verify independently before acting.

Location: impit/src/http3.rs:79 (set_h3_support), triggered from impit/src/impit.rs:535-547

set_h3_support() early-returns when the host key already exists:

pub async fn set_h3_support(&self, host: &String, supports_h3: bool) {
    let mut cache = self.h3_alt_svc.write().await;
    if cache.contains_key(host) {
        return;
    }
    cache.insert(host.to_owned(), supports_h3);
}

In send(), after a non-h3 response the code first calls set_h3_support(&host, false) unconditionally, then, if the response carries Alt-Svc: h3, calls set_h3_support(&host, true):

if !h3 {
    if let Some(h3_engine) = engine_guard.as_ref() {
        h3_engine.set_h3_support(&host, false).await;          // inserts host -> false
        if let Some(alt_svc) = response.headers().get("Alt-Svc") {
            if /* contains h3 */ {
                h3_engine.set_h3_support(&host, true).await;    // dropped: key already exists
            }
        }
    }
}

Because the first false insert wins, the later true update is silently dropped.

Impact: A server that advertises HTTP/3 only via the Alt-Svc response header (not via DNS HTTPS records) is never cached as h3-capable, so HTTP/3 is never used for it on subsequent requests. Alt-Svc-based discovery — one of the two documented discovery mechanisms — is effectively dead code.