Validate runtime processor contracts

[crypt0rr]

What changed

• Centralize single/multiple-wordlist mode validation and reject invalid input for processors 2, 3, 6, 7, and 20.
• Share Markov helper path resolution between execution and dependency checks.
• Make self-test validate the Markov helper and report missing dependencies.
• Add regression coverage for invalid modes, fail-closed behavior, and Markov helper success/failure.
• Document the Markov helper requirements and mark F3 complete in the backlog.

Why

Processors 3, 7, and 20 previously accepted invalid wordlist modes and continued with stale or unset selector state. Job 17 also resolved its helper only when running, so self-test could not detect a missing Markov dependency.

Validation

• make test-smoke
• make test-campaign
• make lint
• Bash syntax and diff checks
• Bash coverage: 100% executable-line and function coverage; baseline satisfied