Correct sponsorship annual value docs for ended contracts

[rhiever]

Summary

• The Sponsorship.annual_value docs said an ended contract's annual value is 0. It often isn't, and can equal the total value, so the docs now say to compare end with the save's date to find running contracts.
• total_value no longer claims a fixed share of contracts where total equals annual times years.
• Docs only. No reader behavior changes.

Refs #11

Checklist

• I added or updated tests for this change. (Docs only; fixture comment updated.)
• This change holds no real data: no save files or fragments, screenshots, or real names, uids or values from any save.
• I derived format knowledge only from my own saves and the game UI (no decompilation, leaked or non-disclosure material, or code from unlicensed parsers).
• uv run pytest, uv run ruff check ., uv run ruff format --check . and uv run pyright pass.

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