bug: enforce required repeatable positionals

[codeforester]

Summary

• track the start index of a repeatable positional separately from earlier positional consumption
• require a required repeatable positional to consume at least one of its own values
• cover direct and table declarations with zero, one, and multiple repeatable values

Validation

• bats lib/bash/cli/tests/lib_cli.bats (23 tests)
• ./tests/validate.sh (579 BATS tests plus all extended gates)

Fixes #344