Treat unusable bulk cycle data as ambiguous

When bulk sample or frequency sources are present, do not silently fall
back to summary SM clock confirmation if the bulk cycle data cannot be
used. Report the clear-gap decision as AMBG with a
bulk_cycle_data_unusable reason instead.

Still allow summary-clock fallback when no bulk sample/frequency sources
are present.

Also update the Unknown summary label to describe the broader set of
input-data failures now counted as UNKNOWN.

python/scripts/nvbench_compare.py

@@ -1427,6 +1427,10 @@ def get_bulk_time_and_cycles(timing):
     return samples, samples * frequencies
 
 
+def has_bulk_time_or_frequency_source(timing):
+    return timing.sample_source is not None or timing.frequency_source is not None
+
+
 def scale_interval(interval, scale):
     if not is_positive_finite(scale):
         return None
@@ -1471,17 +1475,31 @@ def confirm_clear_gap_with_clock_rate(
 
 
 def confirm_clear_gap_with_bulk_cycles(status, ref_timing, cmp_timing, thresholds):
+    has_bulk_sources = has_bulk_time_or_frequency_source(
+        ref_timing
+    ) or has_bulk_time_or_frequency_source(cmp_timing)
+    if not has_bulk_sources:
+        return None
+
     ref_bulk = get_bulk_time_and_cycles(ref_timing)
     cmp_bulk = get_bulk_time_and_cycles(cmp_timing)
     if ref_bulk is None or cmp_bulk is None:
-        return None
+        return make_decision(
+            ComparisonStatus.UNDECIDED,
+            "bulk_cycle_data_unusable",
+            "bulk sample time and frequency data are present but unusable for cycle confirmation",
+        )
 
     _, ref_cycles = ref_bulk
     _, cmp_cycles = cmp_bulk
     ref_cycle_interval = compute_timing_interval_from_samples(ref_cycles)
     cmp_cycle_interval = compute_timing_interval_from_samples(cmp_cycles)
     if ref_cycle_interval is None or cmp_cycle_interval is None:
-        return None
+        return make_decision(
+            ComparisonStatus.UNDECIDED,
+            "bulk_cycle_data_unusable",
+            "bulk cycle intervals could not be constructed",
+        )
 
     cycle_status = compare_intervals_for_clear_gap(
         ref_cycle_interval, cmp_cycle_interval, thresholds
@@ -3152,7 +3170,9 @@ def main() -> int:
         legend_entries = format_reason_legend_entries(stats.reason_legend)
         if legend_entries:
             print(f"  - Reason legend: {'; '.join(legend_entries)}")
-    print(f"  - Unknown     (infinite or unavailable noise): {stats.unknown_count}")
+    print(
+        f"  - Unknown     (input data unavailable or unusable): {stats.unknown_count}"
+    )
     try:
         write_bulk_debug_python(bulk_debug_output, bulk_debug_rows or [])
     except OSError as exc: