Add multi-instance launcher

[darwvin-dev]

Summary

• Add a -multi launcher mode that reads role,count,args CSV rows and starts multiple SIPp child processes.
• Add -multi_base_port plus {role}, {instance}, {base_port}, {instance_port}, and {port} placeholders for generated child arguments.
• Document the CSV format and add unit coverage for parsing, validation, and argument expansion.
• Resolve CodeQL findings by canonicalizing the CSV file path, requiring a regular file, avoiding execvp, and using the current executable path for child launches.

Validation

• Local compile check: g++ -std=c++17 -Wall -Werror -pedantic -Iinclude -c src/multi_instance.cpp -o /tmp/multi_instance.o
• Local Docker Debian build from a fresh build directory: cmake -S /work -B /work/build-upstream -DUSE_GSL=0 -DUSE_SCTP=0 -DUSE_PCAP=0
• Local unit tests: /work/build-upstream/sipp_unittest (61 passed)
• Local binary build: cmake --build /work/build-upstream --target sipp
• Local smoke: ./build-upstream/sipp -multi /tmp/sipp-multi-upstream-smoke.csv -multi_base_port 5070 with 2 UAS + 2 UAC -m 0 children exited successfully.
• GitHub checks passing: lint, build, build-system-gtest, build-osx, build-static, build-wolfssl, codespell, CodeQL Analyze, and CodeQL alerts.