fix: Address PR review findings - resource limits and code cleanup

[inureyes]

Implement range expansion syntax for specifying multiple hosts:

• Simple ranges: node[1-5] -> node1, node2, node3, node4, node5
• Zero-padded: node[01-05] -> node01, node02, node03, node04, node05
• Comma-separated: node[1,3,5] -> node1, node3, node5
• Mixed: node[1-3,7] -> node1, node2, node3, node7
• Cartesian product: rack[1-2]-node[1-3] -> 6 hosts
• File input: ^/path/to/file reads hosts from file

Integrate with -H option, --filter, and --exclude options.
Add 52 unit tests for hostlist module.HIGH Priority Fixes:

• Add resource exhaustion protection in parse_hostfile():
  • Maximum file size limit of 1 MB
  • Maximum line count limit of 100,000 lines
  • Check file size before reading to prevent DoS attacks

MEDIUM Priority Fixes:

• Remove code duplication:

  • Move is_hostlist_expression() and looks_like_hostlist_range()
    from src/main.rs and src/app/nodes.rs to src/hostlist/mod.rs
  • Export functions as public API from hostlist module
  • Update all call sites to use the exported functions

• Remove unused variable:

  • Remove unused 'sign' variable from parse_number() in parser.rs

• Add overflow protection:

  • Use checked_mul() for cartesian product allocations in expander.rs
  • Return RangeTooLarge error if overflow would occur
  • Prevent integer overflow in intermediate calculations

All changes compile successfully and pass tests (cargo test, cargo clippy).