refactor(hidpp): split the channel tests by what they exercise

channel/tests.rs was 1191 lines: 30 tests over five areas of the channel. Each is its own module now — lease, send, raw_and_listeners, send_v20, same_header — and tests.rs keeps the imports and the fixtures more than one area uses (the mock channel helpers, the message builders, the pending-table probes); the lease recorder and the same-header constants moved with the only tests that use them. Every child starts with `use super::*`.

Pure move: no test body changed; 30 tests before and after.