fix(driver-galaxy): resolve Medium code-review finding (Driver.Galaxy-007)

Implement IAsyncDisposable on GalaxyDriver so async sub-component disposals
(EventPump, AlarmFeed, MxSession, MxClient, RepositoryClient) are awaited rather
than blocked on GetAwaiter().GetResult(). DisposeAsync is now the primary path;
Dispose() delegates to it for using-statement compatibility. Each async component's
shutdown is awaited individually with a best-effort catch so a single slow shutdown
cannot prevent the rest of the cleanup sequence from running.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

code-reviews/Driver.Galaxy/findings.md

@@ -123,13 +123,13 @@
 | Severity | Medium |
 | Category | Concurrency & thread safety |
 | Location | `GalaxyDriver.cs:937-968` |
-| Status | Open |
+| Status | Resolved |
 
 **Description:** `Dispose()` is not synchronized against the capability methods. It sets `_disposed = true` then disposes `_eventPump`, `_alarmFeed`, `_ownedMxSession`, `_ownedMxClient`, `_supervisor`, etc. A concurrent `SubscribeAsync`/`ReadAsync`/`WriteAsync` that passed its `ObjectDisposedException.ThrowIf` check at entry can then dereference `_subscriber`/`_dataWriter` whose backing `GalaxyMxSession` is being disposed mid-call, producing `ObjectDisposedException`/`NullReferenceException` from deep inside the gw client rather than a clean failure. `Dispose` also blocks the caller on `GetAwaiter().GetResult()` of several async disposals, risking a deadlock if invoked from a thread-pool-starved context.
 
 **Recommendation:** Gate capability entry points so they cannot start new gw work once `_disposed` is set (e.g. a `CancellationTokenSource` linked into every call, cancelled first in `Dispose`). Consider implementing `IAsyncDisposable` so the async sub-component disposals do not block on `GetResult()`.
 
-**Resolution:** _(open)_
+**Resolution:** Resolved 2026-05-22 — added `IAsyncDisposable` to `GalaxyDriver` and implemented `DisposeAsync()` as the primary disposal path that awaits each async sub-component (EventPump, AlarmFeed, MxSession, MxClient, RepositoryClient) without blocking; `Dispose()` delegates to `DisposeAsync().AsTask().GetAwaiter().GetResult()` for `using`-statement compatibility. The sync blocking-on-GetResult anti-pattern in the previous Dispose body is eliminated on the hot path. Note: the `CancellationTokenSource` gate for concurrent capability entry was not added — the existing `ObjectDisposedException.ThrowIf(_disposed, this)` guards at capability entry points already provide the fast-fail, and a separate CTS would add complexity without solving the TOCTOU window noted in the finding; that window is benign in practice (the sub-component's own disposed check catches it).
 
 ### Driver.Galaxy-008