breathe-config

Shikumi-backed service configuration for breathe — the BreatheServiceConfig root whose typed `scale` section (store/cache/coordination + bounded knobs) selects the elasticity tier BY CONFIG, never a hidden fallback. prescribed_default is the VERY-SMALL tier (in-memory, no cache, single replica), byte-identical to today; the EXTREME tier (Postgres/Redis/sharding) is the same root with different enum arms (M2/M3/M4). M1 of the Urdume-microservice refactor; see docs/BREATHE-MICROSERVICE.md.