Add cxxopts long flags to nec_parse

[tmolteno]

Summary

Replaced the bare argv[1] positional handling in nec_parse with cxxopts — a single-header MIT-licensed C++17 option parser (vendored as antlr/cxxopts.hpp v3.3.1). The header is picked up via the existing -I . include path, so there is no Dockerfile or build_in_docker.sh change.

New options

Long	Short	Behavior
--input <file>	-i	Input NEC file; reads stdin if omitted (fallback preserved)
--output <file>	-o	Output file for the NEC simulation log; stdout if omitted. Structured summary always goes to stdout (matches nec2++)
--benchmark	-b	Runs nec_context::benchmark() and exits
--version	-v	Prints nec_parse <version>
--help	-h	Auto-generated usage (by cxxopts)

The --output FILE* is owned by a unique_ptr with an fclose deleter so it is closed cleanly on every return path. Bad flags produce a clean error message + help text and exit 1 (no uncaught exception).

Why cxxopts (vendored)

• Single MIT-licensed header → GPLv3-compatible, no build-system complexity.
• Vendoring (vs. apt libcxxopts-dev) keeps the antlr Docker side-build self-contained, no new apt dependency, pinned reproducible version.
• Auto-generates --help, supports long+short flags together.

Files changed

• antlr/cxxopts.hpp — new, vendored v3.3.1 (2930 lines)
• antlr/nec_test.cpp — cxxopts parse block, --output wired into s_output.set_file()
• antlr/Makefile — "CLI options (cxxopts)" smoke checks in test:

Verification (all pass)

• --help / -h → usage, exit 0
• --version → nec_parse 2.1.1 [2026-07-18], exit 0
• --benchmark → prints NEC score, exit 0
• --input <file> → full simulation to stdout (unchanged output)
• stdin fallback (no --input) → works
• --output <file> → writes the 182-line simulation log to file
• --bogus → clean error + exit 1
• Build is warning-free; full make test green.

⚠️ Breaking change

Callers using the bare positional form nec_parse <file> must now use nec_parse --input <file> (or -i). The PR #113 simulation behavior is otherwise unchanged.