Immutable blocks with mmap reads

Flush sealed head chunks to ULID-named block directories on disk. Each
block contains CRC'd chunk segment files (mmap'd for reads), a binary
index (symbol table, series, postings with TOC), and a meta.json written
last as the immutability gate. WAL is truncated after block fsync,
preserving the crash-safety ording invariant.

internal/block/meta.go

@@ -0,0 +1,55 @@
+package block
+
+import (
+	"encoding/json"
+	"os"
+	"path/filepath"
+)
+
+// BlockMeta describes a block on disk.
+type BlockMeta struct {
+	ULID       string         `json:"ulid"`
+	MinTime    int64          `json:"minTime"`
+	MaxTime    int64          `json:"maxTime"`
+	Stats      BlockStats     `json:"stats"`
+	Compaction CompactionInfo `json:"compaction"`
+	Version    int            `json:"version"`
+}
+
+// BlockStats holds summary statistics for a block.
+type BlockStats struct {
+	NumSamples int `json:"numSamples"`
+	NumSeries  int `json:"numSeries"`
+	NumChunks  int `json:"numChunks"`
+}
+
+// CompactionInfo records the block's compaction lineage.
+type CompactionInfo struct {
+	Level   int      `json:"level"`
+	Sources []string `json:"sources"`
+}
+
+const metaFilename = "meta.json"
+
+// readMeta reads a block's meta.json from the given block directory.
+func readMeta(dir string) (BlockMeta, error) {
+	data, err := os.ReadFile(filepath.Join(dir, metaFilename))
+	if err != nil {
+		return BlockMeta{}, err
+	}
+	var m BlockMeta
+	if err := json.Unmarshal(data, &m); err != nil {
+		return BlockMeta{}, err
+	}
+	return m, nil
+}
+
+// writeMeta writes a block's meta.json to the given block directory.
+// This is the last file written when creating a block — the immutability gate.
+func writeMeta(dir string, m BlockMeta) error {
+	data, err := json.MarshalIndent(m, "", "  ")
+	if err != nil {
+		return err
+	}
+	return os.WriteFile(filepath.Join(dir, metaFilename), data, 0644)
+}