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.

labels/labels.go

@@ -0,0 +1,8 @@
+// Package labels defines the data model for time-series label pairs.
+package labels
+
+// Label is a name/value pair identifying a time series.
+type Label struct {
+	Name  string
+	Value string
+}