feat(host): PUNKTFUNK_HOST_NAME names the host in Moonlight and the clients

A box called `bazzite-htpc` had no way to present itself as "Living Room"
short of renaming the machine. The new knob overrides the name everywhere
a human sees it: the GameStream serverinfo <hostname> element and the
mDNS service instance name both adverts carry. Unset (the default) is the
machine's own hostname, exactly as before.

Free text is the point, so the DNS-level name is now a separate concern
from the display name. The instance label may contain spaces and accents;
an A-record target may not, and mdns-sd rejects the whole ServiceInfo if
the target is not a legal name — which would take discovery down rather
than merely look wrong. dns_label() sanitizes the target and passes an
already-legal name through byte-for-byte, so hosts without the override
advertise precisely what they always did. The display name loses `.` (it
would split the label, and clients derive the name as the first label of
the fullname, so "Ben's PC v1.2" would arrive as "Ben's PC v1") and is
capped at the 63-byte DNS-SD ceiling on a char boundary.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
(cherry picked from commit bbf72261a12e0e67601f549d40db65ae61979268)

crates/punktfunk-host/src/discovery.rs

@@ -51,6 +51,33 @@ fn mdns_off_value(s: &str) -> bool {
 /// Wire protocol id advertised in the `proto` TXT record.
 pub const NATIVE_PROTO: &str = "punktfunk/1";
 
+/// The DNS label a display name advertises its A record under (`<label>.local.`), which is a
+/// different thing from the service *instance* name: an instance name may be free text
+/// ("Living Room PC" — see `PUNKTFUNK_HOST_NAME`), a host name may not, and `mdns-sd` rejects the
+/// whole `ServiceInfo` if the target isn't a legal name — which would take discovery down entirely
+/// rather than just look wrong. Anything outside `[A-Za-z0-9.-]` becomes `-`; a name that is
+/// already legal (every machine hostname, i.e. every host without the override) passes through
+/// byte-for-byte, so this changes nothing for hosts that don't set the knob.
+pub(crate) fn dns_label(name: &str) -> String {
+    let mut out = String::new();
+    for c in name.trim().chars() {
+        if out.len() >= 63 {
+            break;
+        }
+        out.push(if c.is_ascii_alphanumeric() || c == '-' || c == '.' {
+            c
+        } else {
+            '-'
+        });
+    }
+    let out = out.trim_matches(['-', '.']).to_string();
+    if out.is_empty() {
+        "punktfunk-host".to_string()
+    } else {
+        out
+    }
+}
+
 /// Holds the mDNS daemon; dropping it unregisters the service.
 pub struct Advert {
     _daemon: ServiceDaemon,
@@ -70,7 +97,9 @@ pub fn advertise_native(
     mgmt_port: Option<u16>,
 ) -> Result<Advert> {
     let daemon = ServiceDaemon::new().context("create mDNS daemon")?;
-    let host_name = format!("{hostname}.local.");
+    // `hostname` is the DISPLAY name (the instance label clients read back); the A-record target
+    // has to be a legal DNS name, hence the separate sanitized label.
+    let host_name = format!("{}.local.", dns_label(hostname));
     let mut props: HashMap<String, String> = HashMap::new();
     props.insert("proto".into(), NATIVE_PROTO.into());
     props.insert("fp".into(), fingerprint.to_string());
@@ -116,7 +145,24 @@ pub fn advertise_native(
 
 #[cfg(test)]
 mod tests {
-    use super::mdns_off_value;
+    use super::{dns_label, mdns_off_value};
+
+    #[test]
+    fn dns_label_passes_machine_names_through_and_tames_display_names() {
+        // Every host WITHOUT PUNKTFUNK_HOST_NAME must advertise exactly what it did before.
+        for plain in ["bazzite-htpc", "DESKTOP-1A2B3C", "box.lan", "steamdeck"] {
+            assert_eq!(dns_label(plain), plain);
+        }
+        // A free-text display name becomes a legal label instead of poisoning the record.
+        assert_eq!(dns_label("Living Room PC"), "Living-Room-PC");
+        assert_eq!(dns_label("  Ben's Rig!  "), "Ben-s-Rig");
+        assert_eq!(dns_label("Wohnzimmer-PC ☕"), "Wohnzimmer-PC");
+        // Degenerate input still yields something registerable.
+        assert_eq!(dns_label("***"), "punktfunk-host");
+        assert_eq!(dns_label(""), "punktfunk-host");
+        // DNS caps a label at 63 bytes.
+        assert!(dns_label(&"a".repeat(200)).len() <= 63);
+    }
 
     #[test]
     fn mdns_off_grammar() {