ingotctl, HTTP layer, and self-instrumentation

Add cmd/ingotctl with four subcommands: blocks (list with stats),
inspect (series/postings dump), chunks (decode raw samples by ref), and
fsck (CRC and index integrity validation across all blocks).

Add cmd/ingothttp with JSON query endpoints: /api/v1/query_range
(Prometheus-style matrix response), /api/v1/read (matcher-based read
requests), and /epi/v1/status (DB stats snapshot). Uses JSON instead of
protobuf to maintain zero dependencies.

Add self-instrumentation via the normal Appender path so metrics are
queryable with the same API: ingot_head_series,
ingot_head_chunks_active, ingot_blocks_total, ingot_compactions_total,
ingot_wal_fsync_duration_seconds.

Supporting changes: block.Validate() and block.ReadMeta() exports,
Head.Stats() for series/chunk counts, WAL.LastSyncDuration() with timed
fsync tracking, DB.Stats() for the HTTP status endpoint.

internal/block/validate.go

@@ -0,0 +1,148 @@
+package block
+
+import (
+	"encoding/binary"
+	"fmt"
+	"hash/crc32"
+	"os"
+	"path/filepath"
+
+	"git.dvdt.dev/david/ingot/internal/index"
+)
+
+// ValidationError describes a single integrity issue found during validation.
+type ValidationError struct {
+	Block   string // ULID or directory name
+	Section string // "meta", "index", "chunks"
+	Detail  string
+}
+
+func (e ValidationError) Error() string {
+	return fmt.Sprintf("%s: %s: %s", e.Block, e.Section, e.Detail)
+}
+
+// ReadMeta reads and returns the BlockMeta for a block directory.
+// Exported for use by ingotctl.
+func ReadMeta(dir string) (BlockMeta, error) {
+	return readMeta(dir)
+}
+
+// Validate performs integrity checks on a single block directory.
+// It checks meta.json consistency, index integrity (magic, version, TOC CRC),
+// and CRC validation on every chunk entry.
+func Validate(dir string) []ValidationError {
+	var errs []ValidationError
+	blockName := filepath.Base(dir)
+
+	// 1. Validate meta.json.
+	meta, err := readMeta(dir)
+	if err != nil {
+		errs = append(errs, ValidationError{blockName, "meta", err.Error()})
+		return errs
+	}
+	if meta.Version != 1 {
+		errs = append(errs, ValidationError{blockName, "meta",
+			fmt.Sprintf("unsupported version %d", meta.Version)})
+	}
+	if meta.MinTime > meta.MaxTime {
+		errs = append(errs, ValidationError{blockName, "meta",
+			fmt.Sprintf("minTime %d > maxTime %d", meta.MinTime, meta.MaxTime)})
+	}
+
+	// 2. Validate index file.
+	indexData, err := os.ReadFile(filepath.Join(dir, "index"))
+	if err != nil {
+		errs = append(errs, ValidationError{blockName, "index", err.Error()})
+	} else {
+		_, err := index.NewReader(indexData)
+		if err != nil {
+			errs = append(errs, ValidationError{blockName, "index", err.Error()})
+		}
+	}
+
+	// 3. Validate all chunk segment files — iterate every entry and check CRC.
+	chunksDir := filepath.Join(dir, chunksDirName)
+	entries, err := os.ReadDir(chunksDir)
+	if err != nil {
+		errs = append(errs, ValidationError{blockName, "chunks", err.Error()})
+		return errs
+	}
+
+	for _, e := range entries {
+		if e.IsDir() {
+			continue
+		}
+		segIdx := parseSegmentName(e.Name())
+		if segIdx < 0 {
+			continue
+		}
+
+		data, err := os.ReadFile(filepath.Join(chunksDir, e.Name()))
+		if err != nil {
+			errs = append(errs, ValidationError{blockName, "chunks",
+				fmt.Sprintf("segment %s: %s", e.Name(), err)})
+			continue
+		}
+
+		segErrs := validateChunkSegment(data, e.Name())
+		for _, se := range segErrs {
+			errs = append(errs, ValidationError{blockName, "chunks", se})
+		}
+	}
+
+	return errs
+}
+
+// validateChunkSegment checks every chunk entry in a segment file for CRC integrity.
+func validateChunkSegment(data []byte, name string) []string {
+	var errs []string
+
+	if len(data) < chunkHeaderLen {
+		return []string{fmt.Sprintf("segment %s: too short for header", name)}
+	}
+
+	magic := binary.BigEndian.Uint32(data[:4])
+	if magic != chunkMagic {
+		return []string{fmt.Sprintf("segment %s: invalid magic %#x", name, magic)}
+	}
+	if data[4] != chunkVersion {
+		return []string{fmt.Sprintf("segment %s: unsupported version %d", name, data[4])}
+	}
+
+	off := chunkHeaderLen
+	entryIdx := 0
+	for off < len(data) {
+		if off+chunkEntryHeaderLen > len(data) {
+			errs = append(errs, fmt.Sprintf("segment %s entry %d at offset %d: truncated header",
+				name, entryIdx, off))
+			break
+		}
+
+		dataLen := int(binary.BigEndian.Uint32(data[off : off+4]))
+		encoding := data[off+4]
+		off += chunkEntryHeaderLen
+
+		end := off + dataLen + chunkEntryCRCLen
+		if end > len(data) {
+			errs = append(errs, fmt.Sprintf("segment %s entry %d: truncated data (need %d bytes, have %d)",
+				name, entryIdx, dataLen+chunkEntryCRCLen, len(data)-off))
+			break
+		}
+
+		chunkBytes := data[off : off+dataLen]
+		off += dataLen
+
+		wantCRC := binary.BigEndian.Uint32(data[off : off+4])
+		crc := crc32.New(castagnoliTable)
+		crc.Write([]byte{encoding})
+		crc.Write(chunkBytes)
+		if crc.Sum32() != wantCRC {
+			errs = append(errs, fmt.Sprintf("segment %s entry %d at offset %d: CRC mismatch",
+				name, entryIdx, off-dataLen-chunkEntryHeaderLen))
+		}
+		off += chunkEntryCRCLen
+		entryIdx++
+	}
+
+	return errs
+}