Add memory-based batching support

Implement memory-aware batch sizing with --batch-mem CLI option, enabling adaptive batching based on estimated sequence memory footprint. Key changes:
- Added _BatchMem and related getters/setters in pkg/obidefault
- Implemented RebatchBySize() in pkg/obiter for memory-constrained batching
- Added BioSequence.MemorySize() for conservative memory estimation
- Integrated batch-mem option in pkg/obioptions with human-readable size parsing (e.g., 128K, 64M, 1G)
- Added obiutils.ParseMemSize/FormatMemSize for unit conversion
- Enhanced pool GC in pkg/obiseq/pool.go to trigger explicit GC for large slice discards
- Updated sequence_reader.go to apply memory-based rebatching when enabled

pkg/obiseq/biosequence.go

@@ -273,6 +273,28 @@ func (s *BioSequence) Len() int {
 	return len(s.sequence)
 }
 
+// MemorySize returns an estimate of the memory footprint of the BioSequence
+// in bytes. It accounts for the sequence, quality scores, feature data,
+// annotations, and fixed struct overhead. The estimate is conservative
+// (cap rather than len for byte slices) so it is suitable for memory-based
+// batching decisions.
+func (s *BioSequence) MemorySize() int {
+	if s == nil {
+		return 0
+	}
+	// fixed struct overhead (strings, pointers, mutex pointer)
+	const overhead = 128
+	n := overhead
+	n += cap(s.sequence)
+	n += cap(s.qualities)
+	n += cap(s.feature)
+	n += len(s.id)
+	n += len(s.source)
+	// rough annotation estimate: each key+value pair ~64 bytes on average
+	n += len(s.annotations) * 64
+	return n
+}
+
 // HasQualities checks if the BioSequence has sequence qualitiy scores.
 //
 // This function does not have any parameters.

pkg/obiseq/pool.go

@@ -1,13 +1,20 @@
 package obiseq
 
 import (
+	"runtime"
 	"sync"
+	"sync/atomic"
 
 	log "github.com/sirupsen/logrus"
 
 	"git.metabarcoding.org/obitools/obitools4/obitools4/pkg/obiutils"
 )
 
+const _LargeSliceThreshold = 100 * 1024        // 100 kb — below: leave to GC, above: trigger explicit GC
+const _GCBytesBudget = int64(256 * 1024 * 1024) // trigger GC every 256 MB of large discards
+
+var _largeSliceDiscardedBytes = atomic.Int64{}
+
 var _BioSequenceByteSlicePool = sync.Pool{
 	New: func() interface{} {
 		bs := make([]byte, 0, 300)
@@ -34,6 +41,13 @@ func RecycleSlice(s *[]byte) {
 		}
 		if cap(*s) <= 1024 {
 			_BioSequenceByteSlicePool.Put(s)
+		} else if cap(*s) >= _LargeSliceThreshold {
+			n := int64(cap(*s))
+			*s = nil
+			prev := _largeSliceDiscardedBytes.Load()
+			if _largeSliceDiscardedBytes.Add(n)/_GCBytesBudget > prev/_GCBytesBudget {
+				runtime.GC()
+			}
 		}
 	}
 }