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/wal/wal.go

@@ -3,6 +3,7 @@ package wal
 import (
 	"os"
 	"sync"
+	"sync/atomic"
 	"time"
 )
 
@@ -46,6 +47,8 @@ type WAL struct {
 	segmentOff int64
 	buf        []byte
 
+	lastSyncDur atomic.Int64 // nanoseconds of last fsync
+
 	done chan struct{}
 	wg   sync.WaitGroup
 }
@@ -164,7 +167,21 @@ func (w *WAL) Replay() (*Reader, error) {
 func (w *WAL) Sync() error {
 	w.mu.Lock()
 	defer w.mu.Unlock()
-	return w.segment.Sync()
+	return w.timedSync()
+}
+
+// LastSyncDuration returns the duration of the most recent fsync in seconds.
+func (w *WAL) LastSyncDuration() float64 {
+	ns := w.lastSyncDur.Load()
+	return float64(ns) / 1e9
+}
+
+// timedSync fsyncs the segment and records the duration. Caller must hold w.mu.
+func (w *WAL) timedSync() error {
+	start := time.Now()
+	err := w.segment.Sync()
+	w.lastSyncDur.Store(int64(time.Since(start)))
+	return err
 }
 
 // Truncate deletes all segments with index less than below.
@@ -244,7 +261,7 @@ func (w *WAL) syncLoop(interval time.Duration) {
 			return
 		case <-ticker.C:
 			w.mu.Lock()
-			w.segment.Sync()
+			w.timedSync()
 			w.mu.Unlock()
 		}
 	}