[1862 USA & Canada] PR 04: SR mechanics — GHU discount, NYH par lock, private close triggers

[neutronc]

Fixes #12623

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

Explanation of Change

Two SR/OR mechanic features for 1862 USA & Canada:

GHU token discount + NYH par price lock

• Great Horseshoe Unlocked (GHU) private ability: owning corporation pays $20 less per token placement (min $0). Implemented in Step::Token override.
• NYH corporation must par at exactly $100 (the fixed price on its director share). Step::BuySellParShares blocks other par prices for NYH.

Private company close triggers

• No train-purchase close event in 1862 — privates close on named triggers only.
• SOC closes when both CPR and UP have connected to Salt Lake City (Golden Spike).
• NHSC closes when NYH floats.
• PSC closes when WP pays its first dividend.
• FNY closes when NYC pays its first dividend.
• TOR closes automatically after the Toronto gray tile is placed (handled via closed_when_used_up on its tile-lay ability).
• Step::Dividend override handles first-payout detection for PSC/FNY.

Screenshots

N/A — no tile or map changes.

Any Assumptions / Hacks

None.