ax(mining): replace prose comment with usage example on bufferPool

AX Principle 2: comments show HOW via concrete call, not WHAT via prose.
The previous comment described the variable's purpose in prose — replaced
with a direct usage example showing the Get/Reset/Put pattern.

Co-Authored-By: Charon <charon@lethean.io>

pkg/mining/bufpool.go

@@ -6,8 +6,7 @@ import (
 	"sync"
 )
 
-// bufferPool provides reusable byte buffers for JSON encoding.
-// This reduces allocation overhead in hot paths like WebSocket event serialization.
+// buf := bufferPool.Get().(*bytes.Buffer); buf.Reset(); defer bufferPool.Put(buf)
 var bufferPool = sync.Pool{
 	New: func() interface{} {
 		return bytes.NewBuffer(make([]byte, 0, 1024))