Release 3.8.1.1: lighter idle UI poll, core cache, multicore ping.

Reduce idle CPU (cheap PollUI, binary cache, logbuf cap, DOM fingerprint),
speed ping/AWG HostPort, CopyBuffer pool; ship arm64 build and update notes.

Co-authored-by: Cursor <cursoragent@cursor.com>

internal/logbuf/buffer.go

@@ -0,0 +1,59 @@
+package logbuf
+
+import (
+	"bytes"
+	"sync"
+)
+
+const DefaultMax = 256 << 10 // 256 KiB
+
+// Buffer is a concurrency-safe, size-capped bytes.Buffer replacement for engine stderr.
+type Buffer struct {
+	mu  sync.Mutex
+	buf bytes.Buffer
+	max int
+}
+
+func New(max int) *Buffer {
+	if max <= 0 {
+		max = DefaultMax
+	}
+	return &Buffer{max: max}
+}
+
+func (b *Buffer) Write(p []byte) (int, error) {
+	if b == nil {
+		return len(p), nil
+	}
+	b.mu.Lock()
+	defer b.mu.Unlock()
+	if b.buf.Len()+len(p) > b.max {
+		// Drop oldest half to keep recent diagnostics.
+		keep := b.buf.Bytes()
+		if len(keep) > b.max/2 {
+			keep = keep[len(keep)-b.max/2:]
+		}
+		b.buf.Reset()
+		_, _ = b.buf.Write(keep)
+		_, _ = b.buf.WriteString("\n… log truncated …\n")
+	}
+	return b.buf.Write(p)
+}
+
+func (b *Buffer) String() string {
+	if b == nil {
+		return ""
+	}
+	b.mu.Lock()
+	defer b.mu.Unlock()
+	return b.buf.String()
+}
+
+func (b *Buffer) Len() int {
+	if b == nil {
+		return 0
+	}
+	b.mu.Lock()
+	defer b.mu.Unlock()
+	return b.buf.Len()
+}