perf(scan): parent-first dispatch, smaller load prefix, dedup repeat skipping

cache_options.go

@@ -9,3 +9,17 @@ func WithHunkDedupBudget(bytes int) ScannerOption {
 		hs.hunkDedupBudget = bytes
 	}
 }
+
+// WithHunkDedupRetainedBudget bounds the hunk bytes a deduplicating scan
+// holds between a worker computing them and the consumer callback seeing
+// them. The sequencer stops handing out new pairs while the charge is above
+// this budget, so the bound trades scan throughput for peak memory. The
+// default of 512 MiB keeps every worker busy through a multi-megabyte blob
+// on large hosts. Values below 1 MiB are raised to 1 MiB so a single large
+// hunk can always be admitted.
+func WithHunkDedupRetainedBudget(bytes int) ScannerOption {
+	bytes = max(bytes, 1<<20)
+	return func(hs *HistoryScanner) {
+		hs.dedupLimits.retainedBytesCap = int64(bytes)
+	}
+}