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

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 {

internal/delivery/target_database_test.go

@@ -184,13 +184,20 @@ func TestArchiveWriter_RecreatesAfterRemoval(
 func TestArchiveWriter_ReopenDebounce(t *testing.T) {
 	t.Parallel()
 
-	// A generous debounce keeps the two rapid writes inside
+	const debounce = 2 * time.Second
-	// the window even on a heavily loaded test machine.
+
 	path := filepath.Join(t.TempDir(), "archive-wh.db")
 	w := delivery.NewExportArchiveWriter(
-		path, archiveTestLogger(), 2*time.Second,
+		path, archiveTestLogger(), debounce,
 	)
 
+	// The writer measures its reopen debounce on this clock, which
+	// only the test moves, so how long the host takes between
+	// writes cannot change the result.
+	now := time.Now()
+
+	w.SetNow(func() time.Time { return now })
+
 	require.NoError(t, w.Write(
 		delivery.ExportArchivedEvent{EventID: "a"}, 0,
 	))
@@ -202,7 +209,7 @@ func TestArchiveWriter_ReopenDebounce(t *testing.T) {
 	// initial open — no extra close/reopen.
 	assert.Equal(t, 1, w.Reopens())
 
-	time.Sleep(2100 * time.Millisecond)
+	now = now.Add(debounce)
 
 	require.NoError(t, w.Write(
 		delivery.ExportArchivedEvent{EventID: "c"}, 0,