Add capturing log writer test and expose LogSource (#9)

[itsniper]

Salvages the still-useful parts of the stale `9-automated-testing` branch (rebased onto latest `master`) and drops what the current harness already supersedes.

Relates to #9 (NFR 2 — Automated Testing). Opening against the closed parent issue per request to track this follow-on testing work.

What changed

• Expose `LogSource` — re-export `Willow.LogSource` as a `public typealias` in `LogWriters.swift`. The public `LogWriter.writeMessage(_:logLevel:logSource:)` surface already exposes `LogSource` in its signature, so consumers implementing a custom writer previously had to `import Willow` just to name the type. This matches the sibling re-exports (`LogModifier`, `LogWriter`, etc.).
• Add a capturing-writer test — new `CapturingLogWriter` test helper plus `capturingWriterRecordsForwardedMessagesAndLevels`, which asserts on the exact messages and levels `LoggingService` forwards to its writers (and that disabling halts forwarding). This closes the gap where existing logging tests exercised the code paths for coverage but never verified emitted output at the writer boundary.
• Drop `LoggingServiceMock.swift` — the original branch added a `LoggingService` subclass + mock writer that was never wired to any assertion and is superseded by the established harness pattern.

Verification

• `swift test` — 28/28 pass locally from a plain terminal (headless), ~0.78s.
• Final diff vs `master`: `LogWriters.swift` (+3) and `ReliaBLEManagerTests.swift` (+64); no production/library behavior change.

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