Drive the archive reopen debounce test from a clock (closes #190)

The archive writer now reads the time its reopen debounce is
measured on from a clock field, time.Now outside tests. The
debounce test moves that clock instead of sleeping past a real
2 second window, so a slow host between the two quick writes can
no longer turn a correct result red.

Model: opus-5-5

internal/delivery/target_database_archive.go

@@ -194,6 +194,10 @@ type archiveWriter struct {
 	lastReopen time.Time
 	reopens    int
 
+	// now is the clock the reopen debounce is measured on. It is
+	// time.Now outside tests.
+	now func() time.Time
+
 	// evicted marks a writer that has been removed from the
 	// registry. Its handle is closed and it must never open the
 	// file again: nothing holds it any more, so a reopen would
@@ -228,6 +232,7 @@ func newArchiveWriter(
 		path:     path,
 		log:      log,
 		debounce: archiveReopenDebounce,
+		now:      time.Now,
 	}
 }
 
@@ -263,7 +268,7 @@ func (w *archiveWriter) write(
 		)
 	}
 
-	if time.Since(w.lastReopen) >= w.debounce {
+	if w.now().Sub(w.lastReopen) >= w.debounce {
 		return w.reopen(expiry)
 	}
 
@@ -323,7 +328,7 @@ func (w *archiveWriter) openMode(
 	}
 
 	w.db = gdb
-	w.lastReopen = time.Now()
+	w.lastReopen = w.now()
 	w.reopens++
 
 	if expiry > 0 {