perf: adaptive encoder pool buffer threshold

[xe-nvdk]

Problem

PutEncoder() drops the wbuf write buffer if cap > 32KB. If Arc's messages are consistently 50-100KB, every pooled encoder loses its buffer and must re-grow on next use. This causes repeated allocations and GC pressure under sustained load with large messages.

Proposal

Options:

1. Raise the threshold (e.g., 128KB or 256KB)
2. Make the threshold configurable via SetPoolBufferLimit(n int)
3. Use adaptive sizing: track recent max and retain buffers up to 2x the observed max

Also apply the same pattern to the decoder's buf field (currently 32KB threshold).

Files

• encode.go — PutEncoder() (line ~58)
• decode.go — PutDecoder() (line ~45)

Expected Impact

MEDIUM — eliminates buffer re-growth allocations for workloads with consistently large messages. Reduces tail latency spikes.