erlang: send_after to registered name + gen_server timeout returns (T5+T6, 771/771)

T5 — send_after addresses a registered atom name; the delayed message
lands in that process's mailbox (destination resolved at fire time,
dead/unregistered targets drop silently).

T6 — gen_server loop now handles the {reply,R,S,T} / {noreply,S,T}
timeout-bearing callback returns by scheduling {timeout} to itself via
send_after; handle_info({timeout}, S) fires when no other message
arrives first. Sanity-checks the library hookup.

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

lib/erlang/scoreboard.md

@@ -1,6 +1,6 @@
 # Erlang-on-SX Scoreboard
 
-**Total: 769 / 769 tests passing**
+**Total: 771 / 771 tests passing**
 
 |  | Suite | Pass | Total |
 |---|---|---|---|
@@ -15,7 +15,7 @@
 | ✅ | fib | 8 | 8 |
 | ✅ | ffi | 37 | 37 |
 | ✅ | vm | 78 | 78 |
-| ✅ | send_after | 8 | 8 |
+| ✅ | send_after | 10 | 10 |
 
 
 Generated by `lib/erlang/conformance.sh`.