refactor(device): split the inventory tests by what they exercise

inventory/tests.rs was 1391 lines: 47 tests over seven areas of the enumerator. Each area is its own module now — cache, deferral, unifying, publication, retry, bolt, backfill — and tests.rs keeps the imports and the fixtures more than one area uses (the cache entry, the inventory and receiver builders, the model and probe builders); the Bolt silent-slot fixtures moved with the only tests that use them. Every child starts with `use super::*`; tests.rs imports the ledger module the children reach through super.

Pure move: no test body changed; 47 tests before and after.