feat(settlement): add get_effective_rule(merchant) read

[Userally16]

Closes #579

Adds a public get_effective_rule(merchant) entry point to the Settlement contract, mirroring governance's get_fee_config. The function resolves the full fallback chain used by payment processing: merchant override -> global default -> governance FeeConfig -> bootstrap. Callers no longer need to reconstruct resolution logic manually.

Six tests cover: bootstrap fallback, global default application, merchant-rule override, cleared-rule fallback to global default, unregistered-merchant rejection, and BPS consistency with calculate_fee_split.

Description

Related Issues

Closes #

Type of Change

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• ⚡ Performance improvement
• ♻️ Refactor / Code cleanliness
• 📝 Documentation update
• 🔧 Tooling / CI configuration

Checklist

• My code follows the code style guidelines of this project.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation (if applicable).
• My changes generate no new compiler warnings or lints.
• I have added tests that prove my fix is effective or that my feature works (if applicable).
• All new and existing tests pass locally.

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