[orian]

Summary

• synthesize shared table settings from the key-and-value intersection across all member environments
• compute additive residue patches for every environment, including the selected baseline
• apply the same intersection rule to assignment-driven group layers
• keep group placement fail-closed for differences outside table settings
• document the shared/group settings behavior

This covers settings that are common, present only in a subset, and present everywhere with different values without requiring subtractive patches.

Closes #229

Test plan

• CLI subprocess end-to-end test with real dumps, JSON assignments, generated shared/group/env files, manifests, and built-in round-trip verification
• end-to-end coverage in auto and forced-shared modes
• shared and group settings-intersection coverage in the same production-shaped fixture
• just test
• just coverage (race enabled)
• just vet
• just build
• gofmt and git diff checks