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/index/index.go

@@ -0,0 +1,54 @@
+// Package index implements the binary index file format for ingot blocks.
+//
+// An index file contains four sections:
+//
+//	[Header 5B] [Symbol Table] [Series] [Postings] [TOC 28B]
+//
+// The TOC at the end stores offsets to each section. Readers seek to the
+// end, read the TOC, and use the offsets to locate each section.
+package index
+
+import (
+	"errors"
+
+	"git.dvdt.dev/david/ingot/labels"
+)
+
+const (
+	indexMagic   uint32 = 0x0FACEDB1
+	indexVersion byte   = 1
+	headerLen           = 5  // 4-byte magic + 1-byte version
+	tocLen              = 28 // 3 offsets (24) + CRC (4)
+)
+
+var (
+	ErrInvalidMagic   = errors.New("index: invalid magic number")
+	ErrInvalidVersion = errors.New("index: unsupported version")
+	ErrCorruptTOC     = errors.New("index: corrupt TOC (CRC mismatch)")
+	ErrCorruptIndex   = errors.New("index: corrupt index data")
+	ErrTooShort       = errors.New("index: file too short")
+)
+
+// ChunkRef encodes a chunk's location as (segment << 32 | offset).
+type ChunkRef uint64
+
+func NewChunkRef(segment, offset uint32) ChunkRef {
+	return ChunkRef(uint64(segment)<<32 | uint64(offset))
+}
+
+func (r ChunkRef) Segment() uint32 { return uint32(r >> 32) }
+func (r ChunkRef) Offset() uint32  { return uint32(r) }
+
+// ChunkMeta describes a chunk's time range and location in a chunk file.
+type ChunkMeta struct {
+	MinT int64
+	MaxT int64
+	Ref  ChunkRef
+}
+
+// SeriesEntry is the input for writing and the output for reading a series.
+type SeriesEntry struct {
+	Ref    uint64
+	Labels []labels.Label
+	Chunks []ChunkMeta
+}