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/config.go

@@ -53,7 +53,7 @@ const (
 	DefaultMaxWorkers                 = 10
 	DefaultBufferCapacity             = 512
 	DefaultGCTriggerInterval          = 100
-	DefaultMemoryUsageTrackerInterval = 1 * time.Hour
+	DefaultMemoryUsageTrackerInterval = 5 * time.Minute
 )
 
 // Checkpoints configures periodic persistence of in-memory metric data.