Harden remaining panic surfaces

[gphiIIips]

Summary

This PR hardens the remaining avoidable runtime panic surfaces found after the config fallback work.

Changes:

• Replaces hot-loop HashMap::get(...).unwrap() lookups in rules with canonical list enumeration where the list order is already the source of truth.
• Removes a redundant treatment-failure optional-state unwrap by binding the recorded start level directly.
• Makes weighted mechanism-profile sampling fail closed on mismatched, negative, or non-finite weights instead of indexing past the sampled slot.
• Makes optional journey-logger snapshot locks recover from mutex poison and removes one avoidable active_journeys.get(...).unwrap().
• Extends dimension invariants to cover DrugClass::all, DRUG_CLASS_LOOKUP, and drug_class_for_drug alignment.

The remaining panic!/expect sites after this pass are deliberate fail-fast contract checks: startup/Rayon setup, invalid RNG seed parsing, empty bacteria list, required config lookup, mechanism enum integrity, and misuse of Region::Home without prior resolution.

Validation

• cargo check --all-targets
• cargo test --all-targets -- --nocapture
• cargo test weighted_profile_sampling -- --nocapture
• cargo test --test dimension_invariants -- --nocapture
• git diff --check

Note: whole-repo cargo fmt is currently blocked by pre-existing trailing whitespace in src/simulation/population.rs; this PR intentionally avoids broad formatter churn.