mesh-issues: resume an interrupted carry by revision and fault one issue instead of the whole start, so no kept value is lost and the tracker always starts (review of PR 228)

An open's lost answer is judged by the issue's first entry; an entry that does not
fold faults its issue on start; a value written beside the log is never folded over;
one issue is kept in at most 252 KiB so its whole operation fits a log entry.

modules/mesh-issues/cmd/mesh-issues/tracker_test.go

@@ -196,7 +196,7 @@ func (u unreadableLog) AppendAt(key string, value any, expect uint64) (uint64, e
 	_, _ = u.fakeLog.AppendAt(key, value, expect)
 	return 0, errors.New("no answer in 30 s")
 }
-func (u unreadableLog) Last(string) (*stdio.LogEntry, error) {
+func (u unreadableLog) Read(uint64, string, int) (*stdio.LogPage, error) {
 	return nil, errors.New("no answer in 30 s")
 }
 
@@ -447,7 +447,7 @@ func TestANoteThatWouldMakeTheIssueTooLargeIsRefusedAndNothingChanges(t *testing
 	for i := 0; i < 40 && err == nil; i++ {
 		_, err = h.Note(n, note)
 	}
-	if err == nil || !strings.Contains(err.Error(), "256 KiB") {
+	if err == nil || !strings.Contains(err.Error(), "252 KiB") {
 		t.Fatalf("no refusal: %v", err)
 	}
 	e, _ := h.mem.Get(IssuesState, keyOf(n))