Implement full resolved subject model for multi-person scenarios

[TommiLindfors]

Context

Alpha now includes a deterministic resolved_subject_id in checklist item hash inputs using a minimal bereavement-safe mapping.

Current alpha behavior

Task candidates derive resolved_subject_id from task template target subject role where possible, with a documented fallback for single-subject bereavement scenarios.

Future scope

Implement full subject resolution for multi-person scenarios, including:

• multiple survivors;
• multiple children/dependants;
• subject IDs from scenario facts;
• task-specific subject resolution;
• validation that task templates declare enough target information;
• checklist item IDs that remain stable across multi-subject generation.

Acceptance criteria

• resolved_subject_id is derived from scenario facts where available.
• Multiple subjects can produce separate checklist items without collision.
• Generated explanations identify which person/subject the item relates to.
• Existing alpha single-subject IDs remain stable unless intentionally migrated.