Allow buses .first and .last to be set to a variable length public input.

bus expansion currently assumes buses start and finish empty,

We should add support for setting their first/last values to a variable length public input.

Relevant discussion: https://github.com/0xMiden/air-script/pull/515#discussion_r2549918942

The issue references a discussion in pull request #515. Start by reviewing that discussion to understand the context of bus expansion and the current assumption that buses start and finish empty. Look for code related to bus handling and public inputs in the codebase. Determine where bus first/last values are set and how to modify them to accept variable-length public inputs. Check for existing tests on buses to understand the expected behavior.

Scritto dal modello di indicizzazione a partire dal testo della issue.