[WIP] net/lwip: [EXPERIMENTAL] Add auxiliary lwIP stack for sim.

[elokuucadiaN]

Summary

• Add the complete lwIP 2.0.3 source tree as an optional auxiliary stack. Its BSD license is retained in net/lwip/lwip-2.0.3/COPYING.
• Add the NuttX sys_arch port, Ethernet link adapter, and a raw L2 tap for the simulator network driver. Applications select the prefixed lwip_* API; native NuttX socket routing is unchanged.
• Add sim:lwip configuration and documentation, including the requirement to initialize lwIP from a long-lived context.
• This initial integration is simulator-only and experimental. The tests use three local nuttx-apps programs that are not included in this NuttX PR; there is no dependent apps PR.

Impact

• New feature: Yes, behind CONFIG_NET_LWIP, currently limited to ARCH_BOARD_SIM && NET_PKT.
• Existing users and compatibility: No change when the option is disabled. With the option enabled, applications must explicitly use the lwip_* API and initialize the auxiliary stack in a persistent context.
• Build: Adds the optional lwIP sources and sim:lwip configuration; no default configuration is changed.
• Hardware: Simulator Ethernet only. No physical board or hardware driver is changed.
• Documentation: Documentation/components/net/lwip.rst is included.
• Security: A second TCP/IP stack and raw link-layer tap increase the enabled build's network attack surface; this experimental option is disabled by default.

Testing

• Build host: WSL2 Ubuntu 22.04, x86_64, GCC 11.4.0.

• Target: sim:lwip, NuttX master base bc116157329946d2f7e6c82eb4b2d82009a41ebb; fresh export from the staged Git tree, make olddefconfig && make -j8 completed successfully.

• Local test setup: BOARDCTL_FINALINIT disabled and the three local test programs enabled only in the isolated build configuration. The local NSH test harness calls nuttx_lwip_initialize() from its persistent context. These test-only apps and configuration changes are not included in this PR.

• Runtime: Within one NSH session over a TAP interface (10.0.1.1/24), the host receiver logged:

  UDP ('10.0.1.3', 49153): 16 bytes, b'hello from lwip\n'
  TCP ('10.0.1.3', 49153): b'hello from lwip tcp\n'
  TCP ('10.0.1.3', 49153): received 20 bytes in 1.009 s
  TCP ('10.0.1.3', 49154): received 67108864 bytes in 4.697 s
  tcpbench: total=67108864 bytes time=4.696763 sec throughput=114.307 Mbps
  

• NuttX nxstyle completed with no diagnostics for the ten NuttX-written/modified C and header files. The complete vendored lwIP tree is preserved verbatim; git diff --check reports pre-existing trailing whitespace in third-party files.

• No physical hardware runtime result is available: the current link adapter only supports sim. This is a draft for architecture and scope discussion, not a request to merge without meeting the project's testing requirements.

PR verification self-check

• One functional change: optional auxiliary lwIP integration.
• Required scope, impact, testing limitations, and documentation described.
• All contribution requirements are satisfied; hardware testing and third-party lint disposition remain open.
• Work in progress; keep this PR as a draft.
• Ready for review and merge.

[elokuucadiaN]

Superseded by #20160 after renaming the source branch. The signed commit and test scope are unchanged.