Short, hot-path string primitive for the axess workspace. ShortString is optimized for the workload of short identifiers that are hashed, compared, and cloned at high volume: event taxonomy tags, factor names, routing discriminators. The current internal representation is a placeholder (heap Box<str> + &'static str) suitable for getting the API contract in place; it will be swapped to an Umbra-style 16-byte representation in a follow-up commit. The public API is stable across the swap.