Segmented write-ahead log

Append-only WAL with CRC32-validated records, automatic segment rotation
at 128 MiB, background periodic fsync, and crash recovery by truncation
at the first corrupt or incomplete record.

Record format: type(1) | len(4) | payload | crc32c(4). Two payload
types: series (ref + labels) and samples (batch of ref/t/v). Fixed-width
encoding througout.

Torn-write harness truncates at every byte offset and asserts recovery
produces a valid prefix of the original sequnce, both single-segment and
multi-segment.

DESIGN.md, NOTICE.md, and README.md.

internal/wal/segment.go

@@ -0,0 +1,63 @@
+package wal
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"sort"
+	"strconv"
+)
+
+const (
+	defaultSegmentMaxSize = 128 * 1024 * 1024 // 128 MiB
+	segmentNameLen        = 8                  // "00000001"
+)
+
+// segmentFileName returns the zero-padded filename for a segment index.
+func segmentFileName(index int) string {
+	return fmt.Sprintf("%0*d", segmentNameLen, index)
+}
+
+// parseSegmentIndex parses a segment filename back to its index.
+// Returns -1 if the name is not a valid segment file.
+func parseSegmentIndex(name string) int {
+	if len(name) != segmentNameLen {
+		return -1
+	}
+	n, err := strconv.Atoi(name)
+	if err != nil {
+		return -1
+	}
+	return n
+}
+
+// listSegments returns the sorted indices of all segment files in dir.
+func listSegments(dir string) ([]int, error) {
+	entries, err := os.ReadDir(dir)
+	if err != nil {
+		return nil, err
+	}
+
+	var indices []int
+	for _, e := range entries {
+		if e.IsDir() {
+			continue
+		}
+		idx := parseSegmentIndex(e.Name())
+		if idx >= 0 {
+			indices = append(indices, idx)
+		}
+	}
+	sort.Ints(indices)
+	return indices, nil
+}
+
+// segmentPath returns the full path for a segment index within dir.
+func segmentPath(dir string, index int) string {
+	return filepath.Join(dir, segmentFileName(index))
+}
+
+// createSegment creates a new segment file and returns it open for writing.
+func createSegment(dir string, index int) (*os.File, error) {
+	return os.OpenFile(segmentPath(dir, index), os.O_WRONLY|os.O_CREATE|os.O_EXCL, 0644)
+}