Add context-managed realtime packet session with RT-kernel mode

[Copilot]

This introduces a reusable realtime send/receive session API for Scapy and adds an explicit rt_kernel=True mode for lower-jitter, deterministic I/O behavior on real-time kernels. The goal is to support interactive packet workflows with clear socket lifecycle, configurable latency controls, and RT-safe integration points.

• API: context-managed realtime session

  • Added RealtimeSession and realtime_session(...) in scapy/sendrecv.py.
  • Exposes session-scoped send(), recv(timeout=...), and sendrecv(...).
  • Keeps one socket open across operations and guarantees cleanup on context exit.

• Execution model split by mode

  • Default mode: background receive thread + bounded queue for asynchronous consumption.
  • rt_kernel=True: single-threaded blocking receive path on the same socket to reduce scheduler jitter.

• RT tuning + integration hooks

  • Added optional knobs: socket_rcvbuf, socket_sndbuf, poll_interval, queue_size, strict_drop.
  • Added setup_callback / teardown_callback hooks so callers can apply external process-level RT setup (e.g., scheduler/affinity) outside Scapy core.
  • Preserves kernel-derived packet timestamps when provided by socket paths; only falls back when absent.

• Docs: realtime-kernel caveats

  • Added usage section in doc/scapy/usage.rst covering API usage, mode behavior, tuning options, and RT operational caveats (privileges, starvation risk, latency-vs-completeness tradeoff).

• Regression coverage

  • Added test/regression.uts scenarios for:
    • timeout behavior in RT mode,
    • timestamp ordering,
    • background-thread shutdown semantics,
    • strict-drop freshness policy,
    • session lifecycle (send() requires entered context) and sendrecv() flow.

from scapy.sendrecv import realtime_session

with realtime_session(iface="eth0", rt_kernel=True, poll_interval=0.001) as rt:
    rt.send(pkt)
    ans = rt.recv(timeout=0.05)