feat: add buffer config

[dankeboy36]

This PR is mainly to start a discussion with the Arduino CLI team about whether handler-level buffering is the right approach. I do not really know these things, but I would like to use them downstream. Thank you!

What

• Adds opt-in buffering on the port→TCP path.
• Default behavior stays unbuffered.
• Uses a small queue with an overflow policy (drop or bounded wait).
• No API break: controlled via CONFIGURE keys.

CONFIGURE keys (defaults)

• _buffer.hwm n>=1 (1) – size trigger
• _buffer.interval_ms ms>=0 (0) – time trigger (0 = disabled)
• _buffer.line true|false|on|off|0|1 (false) – flush on \n
• _buffer.queue n>=1 (32) – queued payloads between aggregator and TCP writer
• _buffer.overflow <drop|wait> (drop) – on full queue
• _buffer.overflow_wait_ms ms>=0 (0) – wait budget when overflow=wait

Ref: #33
Arduino CLI: arduino/arduino-cli#2975