fix: Fix metricstore memory explosion from broken emergency free and batch aborts

- Fix MemoryUsageTracker: remove premature bufferPool.Clear() that prevented
  mem.Alloc from decreasing, replace broken ForceFree loop (100 iterations
  with no GC) with progressive time-based Free at 75%/50%/25% retention,
  add bufferPool.Clear()+GC between steps so memory stats update correctly
- Enable debug.FreeOSMemory() after emergency freeing to return memory to OS
- Add adaptive ticker: 30s checks when memory >80% of cap, normal otherwise
- Reduce default memory check interval from 1h to 5min
- Don't abort entire NATS batch on single write error (out-of-order timestamp),
  log warning and continue processing remaining lines
- Prune empty levels from tree after free() to reduce overhead
- Include buffer struct overhead in sizeInBytes() for more accurate reporting

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Entire-Checkpoint: 7ce28627fc1d

pkg/metricstore/buffer.go

@@ -408,3 +408,12 @@ func (b *buffer) count() int64 {
 	}
 	return res
 }
+
+// bufferCount returns the number of buffer nodes in the linked list.
+func (b *buffer) bufferCount() int64 {
+	var n int64
+	for ; b != nil; b = b.prev {
+		n++
+	}
+	return n
+}