Prepare v0.9.0: fix build, harden inputs, add e2e + release workflow

[gistrec]

Summary

Prepare v0.9.0: fix the broken make program build (linker error never
caught because CI only ran make gtests), validate CLI inputs, add a release
workflow and end-to-end tests, and bump cxxopts to v3.3.1.

Changes

• Build. inline Utils helpers to fix duplicate-symbol link error; CI now
  builds the binary on every platform; -std=c++17 -O2 -Wall -Wextra.
• Hardening. Validate --mtu / --port / --bind-port / --ttl /
  --type / --broadcast; cap announced file size at 4 GiB; inet_pton
  instead of inet_addr; int (not char) for Winsock options; bail out
  cleanly on FINISH without NEW_PACKET; check output.write and sendto;
  nothrow allocations; WSACleanup on every error path; fix signed overflow
  on > 2 GiB files.
• CLI. New --bind-port (separates listen port from destination port),
  --version, --help.
• Release. .github/workflows/release.yml builds Linux / macOS arm64 /
  Windows binaries on v* tags and attaches them to a GitHub Release.
• Tests. tests/e2e.sh runs sender + receiver on loopback, verifies
  bit-identical round-trip for 50 KiB and 2 MiB files; CI runs it on Linux
  and macOS (skipped on Windows — Winsock SO_REUSEADDR semantics).
• Dependencies. cxxopts v2.1.1 → v3.3.1.
• Docs. Updated README (new params, Examples, Limitations, C++17 build
  instructions).