feat(examples): add ACF-LIN talker/listener example

[SoundMatt]

Closes #21

Summary

Adds a pair of Linux example applications for the ACF-LIN PDU format (IEEE Std. 1722-2016, chapter 9.4.5):

File	Description
examples/acf-lin/acf-lin-talker.c	Sends simulated LIN frames at 1 Hz over Ethernet or UDP
examples/acf-lin/acf-lin-listener.c	Receives ACF-LIN frames and prints bus ID / identifier / payload
examples/acf-lin/CMakeLists.txt	Build rules (mirrors hello-world pattern)

examples/CMakeLists.txt gains an add_subdirectory(acf-lin) inside the existing Linux-only block.

Design

Since LIN has no standard Linux socket API (unlike SocketCAN for CAN), the talker simulates LIN data — an incrementing single-byte counter — sent periodically. Both tools follow the CLI conventions of the other examples:

Talker options

• -t/--tscf — use TSCF instead of NTSCF
• -u/--udp — use UDP instead of Ethernet
• -i/--ifname IFNAME — Ethernet interface
• -d/--dst-addr MACADDR — destination MAC
• -n/--dst-nw-addr HOST:PORT — UDP destination
• --lin-bus-id N — LIN bus ID (0-31, default 0)
• --lin-id N — LIN frame identifier (0-63, default 1)

Listener options

• -u/--udp, -i/--ifname, -d/--dst-addr, -p/--udp-port

Test plan

# Build
cmake -B build && cmake --build build --target acf-lin-talker acf-lin-listener

# Run on loopback (two terminals)
./build/examples/acf-lin/acf-lin-listener -i lo -d 00:00:00:00:00:00
./build/examples/acf-lin/acf-lin-talker   -i lo -d 00:00:00:00:00:00 --lin-id 5

# UDP mode
./build/examples/acf-lin/acf-lin-listener -u -p 17220
./build/examples/acf-lin/acf-lin-talker   -u -n 127.0.0.1:17220

[nayakned]

Hi @SoundMatt

Thank you for your pull request and interest in incorporating LIN examples into the Open1722 project.
However, I see that the LIN simulator replicates the transmission and reception of LIN frames.
This type of simulation is already done using the VSS listener and talker, for example. The idea of the issue (perhaps not clearly articulated there) is to integrate a real LIN driver in the talker/listener.

There is apparently LIN support in Linux kernel: https://lwn.net/Articles/916049/. Or one can use https://github.com/lin-bus/linux-lin, analogous to the socketCAN.
It would be great if we can work with real LIN frames rather than the values simulated/printed over the console which does not add a lot of value to the examples in my opinion. So I would hold off on this PR.

Currently none of us are actively looking into LIN options in Linux. Would you be interested in this, @SoundMatt?