[Feature]: [P3] Group the GameRules interface past ~12 methods (parked with threshold)

Imported from the engineering backlog (_bmad-output/implementation-artifacts/backlog.md, story seam-grouping).
Priority P3 (do-first order) | Severity: n/a | Status: parked | Epic: sim-split party | Type: design-decision | Recorded: 2026-07-08

Problem / motivation

Parked with a threshold. If GameRules grows past roughly a dozen methods, GROUP the interface (rules.economy.*, rules.pricing.* style) instead of letting it flatten out; a ModernSimulation subclass was steelmanned and ruled OUT (polymorphic deserialize would let a forged save pick its attack surface before validation; inheritance ships Classic bugfixes into Modern silently; the modes disagree rather than stack). Provenance: sim-split party 2026-07-08, Story 1.7 in epics-classic-modern-roadmap-2026-07-08.md (the tripwire test that enforces the seam law).

Start with _bmad-output/implementation-artifacts/backlog.md and the referenced epics-classic-modern-roadmap-2026-07-08.md, then locate the GameRules interface and its tripwire test. Confirm whether the interface has crossed the roughly dozen-method threshold and review the recorded grouping seam decision. Done means the threshold decision is resolved and the enforcing test reflects the agreed structure.