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

The archive writer's reopen debounce test made two writes that had to land inside the real 2-second window, then slept 2.1 seconds to cross it, so a slow host could turn correct code red. The archive writer now reads the time for its reopen debounce from a clock field, time.Now in production, and the test moves that clock instead of sleeping: two writes at one instant open the file once, and a write one debounce later closes and reopens it once. Removing the debounce check fails the test. This was the last test whose result depended on real elapsed time.

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