Function is_semantic_option_key 

pub fn is_semantic_option_key(key: &str) -> bool

Returns true if key is a recognised semantic table-option key.

Two acceptance rules: membership in the closed SEMANTIC_OPTION_KEYS whitelist, OR the open entity sub-namespace (is_entity_option_key, validated by prefix + shape). Everything else under SEMANTIC_PREFIX is rejected, and the internal SEMANTIC_PER_TABLE_INDEX_KEY (outside the prefix) never matches.