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/obidefault/batch.go

@@ -24,3 +24,33 @@ func BatchSize() int {
 func BatchSizePtr() *int {
 	return &_BatchSize
 }
+
+// _BatchMem holds the maximum cumulative memory (in bytes) per batch when
+// memory-based batching is requested. A value of 0 disables memory-based
+// batching and falls back to count-based batching.
+var _BatchMem = 0
+var _BatchMemStr = ""
+
+// SetBatchMem sets the memory budget per batch in bytes.
+func SetBatchMem(n int) {
+	_BatchMem = n
+}
+
+// BatchMem returns the current memory budget per batch in bytes.
+// A value of 0 means memory-based batching is disabled.
+func BatchMem() int {
+	return _BatchMem
+}
+
+func BatchMemPtr() *int {
+	return &_BatchMem
+}
+
+// BatchMemStr returns the raw --batch-mem string value as provided on the CLI.
+func BatchMemStr() string {
+	return _BatchMemStr
+}
+
+func BatchMemStrPtr() *string {
+	return &_BatchMemStr
+}