Adds flagspec_t descriptor table, getopt_long option-table builder, and
the shared stderr-routed rejection path that every flag-accepting
command will migrate onto in later tasks. TDD'd via test_flagspec.c
(1273/1273 controller suite passing).
Task 1.1 of .kiro/specs/xce-test-cli-ux/tasks.md.
Requirements: 2.1, 2.2, 2.3, 2.4, 3.1
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019jaMf9QEwpugH2W9KAuc7f
Rongsong Shen
0748f10cc6