buzz-db: move lib.rs tests (~4,000 lines) to domain modules #17

lib.rs contains 47 test functions spanning ~4,000 lines (lines 5481–9491). These tests cover specific domains — addressable replacement, parameterized replacement, community lookup, routing predicates, pool stats. When the impl Db methods move to domain modules (#2), the tests that exercise those methods should move with them. Community tests → community.rs, replacement tests → event.rs, routing tests stay in lib.rs. Priority: Medium — naturally done alongside #2.

🤖 AI review update (2026-08-23)

Fold this into each #2 domain slice rather than executing it as an independent test-moving PR. Tests should move with the implementation they cover, preserving database isolation and making each refactor reviewable. Leave only genuinely cross-cutting pool/routing tests in lib.rs.