Audit and repair broken or dead Model APIs

[davorg]

Problem

Succession::Model exposes some APIs that appear to be unused or cannot work as written:

• get_succession calls sovereign and succession, but the model has no such attributes.
• sovereign_on_date and succession_on_date default to $self->date, but the model has no date attribute.
• Succession::App still delegates get_succession, although no active caller was found.

These are latent failures and make the model's public API misleading.

Proposed work

• Confirm callers across the application and utility scripts.
• Remove dead APIs and delegation, or repair them if they are intentionally public.
• Make date requirements explicit rather than relying on a nonexistent fallback.
• Update the Model POD to match the resulting API.
• Add focused tests for every retained method.

Acceptance criteria

• No model method relies on nonexistent attributes.
• Dead delegation is removed.
• Retained succession methods have tests covering their supported invocation forms.
• The full test suite passes.