export-runner: classify a completed-but-empty backtest as a data hole, not a failure

The first full production run aborted on Copper 2017-04: a month with no data
*inside* the symbol's range. cTrader does not emit the clean "No historical data"
message for an in-range hole; it runs the backtest to its `}` summary but with a
degenerate empty result and no EXPORT_SUCCESS. run_one misread that as a transient
failure and hard-aborted after retries.

Reaching the `}` summary means the backtest completed cleanly, so the absence of an
export marker is an empty month (rc 3, skipped), not a transient failure. A genuine
transient failure never reaches `}`, which is what distinguishes the two.

scripts/export-runner.sh

@@ -109,9 +109,11 @@ run_one() {
     docker rm -f "$C_NAME" >/dev/null 2>&1
     CURRENT_CONTAINER=""
 
-    if   (( saw_success )); then return 0
-    elif (( saw_empty ));   then return 3
-    else                         return 2   # saw_error, no terminator (timeout/stream died), or `}` without a verdict
+    if   (( saw_success )); then return 0   # DATA_FOUND / EXPORT_SUCCESS
+    elif (( saw_error ));   then return 2   # bot raised an exception (e.g. write failure) -> retry/abort
+    elif (( saw_empty ));   then return 3   # console: "No historical data for the specified period"
+    elif (( saw_term ));    then return 3   # backtest reached its `}` summary but exported nothing -> data hole / empty month
+    else                         return 2   # no terminator: timeout / crash / dead stream -> transient failure
     fi
 }