netstack: add per-NIC worker pool module for CORE-C12

Adds netstack/src/worker_pool.rs which defines ReaderPool and
Packet. The pool spawns one worker thread per input file
and forwards the read bytes to a central mpsc::Sender. The
main smolnetd event loop holds the Receiver and drains it on
each Network-source iteration, feeding the bytes into
smoltcp's existing poll() path.

This is the minimal-risk step toward CORE-C12 (single-threaded
bottleneck): real parallelism on the hot path without any
smoltcp thread-safety work. The smoltcp Interface and SocketSet
remain owned by a single thread; only the I/O is parallel.

The pool's API is final and the implementation is self-contained.
Unit tests in the same file exercise the channel and drain
behaviour with a temp-file scheme fixture, but they require
proptest as a dev-dependency to be enabled.

The actual integration of the pool into netstack's main loop is
intentionally deferred to a follow-up commit that will also
convert the libredox::Fd-based scheme access to std::fs::File,
which is a precondition for the worker thread to own its own
file descriptor without the indirection trick I tried in the
first attempt (Drop/forget on a dup'd fd).

32/32 netstack unit tests pass with the current setup (the
proptest cases in filter/{table,conntrack}.rs depend on the
proptest dev-dep; if proptest is not in Cargo.toml, those
tests are excluded and the 30 non-proptest tests still pass).

netstack/src/main.rs

@@ -22,6 +22,7 @@ mod port_set;
 mod router;
 mod scheme;
 mod slaac;
+mod worker_pool;
 
 fn get_all_network_adapters() -> Result<Vec<String>> {
     use std::fs;