comtrade 0.1.0

• Initial CRAN release.
• Core data retrieval: ct_trade() for goods, ct_services() for services.
• Reference tables: ct_reporters(), ct_commodities(), ct_available().
• Trade analytics: ct_balance(), ct_top_products(), ct_top_partners(), ct_rca(), ct_hhi(), ct_growth(), ct_share(), ct_compare().
• Classification concordance: ct_concordance() with built-in HS/SITC/BEC mapping.
• Key management: ct_set_key(), ct_cache_clear().
• Works without an API key (preview endpoint). Free key available for full access.
• 16 exported functions, 5 dependencies (cli, httr2, stats, tools, utils).