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/obiutils/memsize.go

@@ -0,0 +1,85 @@
+package obiutils
+
+import (
+	"fmt"
+	"strconv"
+	"strings"
+	"unicode"
+)
+
+// ParseMemSize parses a human-readable memory size string and returns the
+// equivalent number of bytes. The value is a number optionally followed by a
+// unit suffix (case-insensitive):
+//
+//	B  or (no suffix) — bytes
+//	K  or KB           — kibibytes  (1 024)
+//	M  or MB           — mebibytes  (1 048 576)
+//	G  or GB           — gibibytes  (1 073 741 824)
+//	T  or TB           — tebibytes  (1 099 511 627 776)
+//
+// Examples: "512", "128K", "128k", "64M", "1G", "2GB"
+func ParseMemSize(s string) (int, error) {
+	s = strings.TrimSpace(s)
+	if s == "" {
+		return 0, fmt.Errorf("empty memory size string")
+	}
+
+	// split numeric prefix from unit suffix
+	i := 0
+	for i < len(s) && (unicode.IsDigit(rune(s[i])) || s[i] == '.') {
+		i++
+	}
+	numStr := s[:i]
+	unit := strings.ToUpper(strings.TrimSpace(s[i:]))
+	// strip trailing 'B' from two-letter units (KB→K, MB→M …)
+	if len(unit) == 2 && unit[1] == 'B' {
+		unit = unit[:1]
+	}
+
+	val, err := strconv.ParseFloat(numStr, 64)
+	if err != nil {
+		return 0, fmt.Errorf("invalid memory size %q: %w", s, err)
+	}
+
+	var multiplier float64
+	switch unit {
+	case "", "B":
+		multiplier = 1
+	case "K":
+		multiplier = 1024
+	case "M":
+		multiplier = 1024 * 1024
+	case "G":
+		multiplier = 1024 * 1024 * 1024
+	case "T":
+		multiplier = 1024 * 1024 * 1024 * 1024
+	default:
+		return 0, fmt.Errorf("unknown memory unit %q in %q", unit, s)
+	}
+
+	return int(val * multiplier), nil
+}
+
+// FormatMemSize formats a byte count as a human-readable string with the
+// largest unit that produces a value ≥ 1 (e.g. 1536 → "1.5K").
+func FormatMemSize(n int) string {
+	units := []struct {
+		suffix string
+		size   int
+	}{
+		{"T", 1024 * 1024 * 1024 * 1024},
+		{"G", 1024 * 1024 * 1024},
+		{"M", 1024 * 1024},
+		{"K", 1024},
+	}
+	for _, u := range units {
+		if n >= u.size {
+			v := float64(n) / float64(u.size)
+			if v == float64(int(v)) {
+				return fmt.Sprintf("%d%s", int(v), u.suffix)
+			}
+			return fmt.Sprintf("%.1f%s", v, u.suffix)
+		}
+	}
+	return fmt.Sprintf("%dB", n)
+}