logger: logger.log becomes the completeness receipt

Its only content was the redundant announce line — the useful shutdown
marker was written AFTER the final drain, so it reached serial and the
ring but never a file, and a truncated boot could only be inferred from
what was missing. The marker now enters the ring BEFORE the drain, so
the drain carries it into logger.log: a directory whose logger.log ends
with 'shutting down; final flush' is complete through shutdown; one
without it was cut early. The sequence-count epilogue stays serial-only,
after the drain, by design.

system/services/logger/logger.zig

@@ -112,9 +112,14 @@ fn onNotification(badge: u64) void {
 }
 
 fn onTerminate() void {
-    // Final drain: everything still in the ring, then close (= flush) all files.
+    // The completeness receipt FIRST: this record enters the ring before the
+    // final drain, so the drain carries it into logger.log — a directory whose
+    // logger.log ends with this marker is complete through shutdown; one that
+    // doesn't was cut early and may be missing tails.
+    _ = system.write("logger: shutting down; final flush\n");
     drain();
     closeAll();
+    // Serial-only epilogue (after the drain, so it reaches no file — by design).
     var line: [96]u8 = undefined;
     _ = system.write(std.fmt.bufPrint(&line, "logger: flushed through sequence {d}\n", .{next_expected_sequence}) catch return);
 }