[18Cuba] Par minors with concession exchange

[chrstas]

Implements 18Cuba-specific logic for parring minor corporations via concessions. Overrides process_par to handle minors differently: sets the corporation’s par, exchanges a concession for the minor’s starting shares, applies any exchange discounts, updates player ownership, closes the concession if needed, and tracks the action. Also adjusts share-buying rules to prevent minors from selling additional shares after floating.

Before clicking "Create"

• Branch is derived from the latest master
• Add the pins or archive_alpha_games label if this change will break existing games
• Code passes linter with docker compose exec rack rubocop -a
• Tests pass cleanly with docker compose exec rack rake

Implementation Notes

• Variable name changed to meaningful name in simple_draft (pending reveiwer comment from last PR)
• Entities consistently completed with types

Explanation of Change

Screenshots

Any Assumptions / Hacks

[chrstas]

Did additional tests for cases with players with low cash but concession. To my understanding it should be covered now.