fix(inject): make the gamescope EIS injector reconnect robustly across sessions

Root cause of "input doesn't work" on the unified host: a single fresh session
injects fine (EIS connects, "Gamescope Virtual Input" device added), but the
host-lifetime injector reused a STALE per-session EIS socket across sessions →
"connect EIS socket …: Connection refused". (Headless gamescope is EIS-only — it
ignores uinput — so libei/EIS is the one input path for both gamescope and KWin;
no second path needed.)

- connect_socket_file: re-READ the relay file and RETRY the connect on
  refused/missing (the live gamescope's EIS appears shortly), bounded at 15s,
  instead of connecting once and bubbling ECONNREFUSED.
- GamescopeProc::drop: clear the relayed EIS socket name on teardown so a dead
  session can't hand a stale path to the next reconnect.

Validated: two sessions back-to-back each reconnect (EIS connected + device added).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/punktfunk-host/src/vdisplay/gamescope.rs

@@ -257,6 +257,9 @@ impl Drop for GamescopeProc {
     fn drop(&mut self) {
         let _ = self.0.kill();
         let _ = self.0.wait();
+        // Clear the relayed EIS socket name so the host-lifetime injector can't reconnect to this
+        // now-dead session's socket between sessions (the stale path is the "Connection refused").
+        let _ = std::fs::remove_file(EI_SOCKET_FILE);
     }
 }