feat(w7): cosmetic room labels carried in the ticket (P5)

Rooms can now be named. A "Room name (optional)" field on the home Create card
mints a ticket carrying the label; PeerSpeakTicket gains a #[serde(default)]
`name` field (backward/forward compatible — serde ignores unknown fields and
defaults missing ones, so old/new builds still interoperate, just without
labels). restamp preserves the label so member-issued doors keep it; new
label_of helper reads it. Every member (creator or joiner) sets current_room.name
from the ticket, so presence reports a consistent "in <name>" to friends, and the
room-screen header shows the label under the wordmark. Labels are sanitized via
sanitize_name on both mint and display (untrusted peer-supplied ticket).

CoreCommand::Join gains room_name (used only when creating). +2 ticket tests
(label round-trip through restamp/label_of, pre-label backward-compat). clippy
--all-targets clean, 257 lib tests green.

Pure seam unit-tested + home field screenshot-verified; the in-room header label
and friend-side "in HangOut" presence display need a live/2-machine confirm.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/presence.rs

@@ -145,7 +145,12 @@ mod tests {
     }
 
     fn valid_ticket(for_id: EndpointId) -> String {
-        PeerSpeakTicket { host_addr: EndpointAddr::from(for_id), topic_id: [9u8; 32] }.to_string()
+        PeerSpeakTicket {
+            host_addr: EndpointAddr::from(for_id),
+            topic_id: [9u8; 32],
+            name: String::new(),
+        }
+        .to_string()
     }
 
     #[test]