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/export_test.go

@@ -512,6 +512,12 @@ func (e *ExportArchiveWriter) Reopen(
 	return e.w.reopen(expiry)
 }
 
+// SetNow replaces the clock the writer measures its reopen
+// debounce on.
+func (e *ExportArchiveWriter) SetNow(now func() time.Time) {
+	e.w.now = now
+}
+
 // Reopens reports how many times the file has been opened.
 func (e *ExportArchiveWriter) Reopens() int {
 	return e.w.reopens