fix(galaxy): park data-change events that overtake their SubscribeBulk reply (#859)

mxaccessgw gives no reply-before-event ordering: the worker completes the STA
command via a RunContinuationsAsynchronously TCS, so the reply is framed on a
thread-pool continuation while the STA already pumps the OnDataChange Advise
queued (drained/framed by a separate loop), and the reply (unary Invoke) and
the event (StreamEvents) are separate HTTP/2 streams. The driver then also
registers handles concurrently with the pump's dispatch loop. MXAccess's
initial value could therefore be dropped as "stale after unsubscribe",
leaving the node at its pre-subscribe value until the next change.

SubscriptionRegistry gains an early-event window: every SubscribeBulk call
site (SubscribeAsync, ResubscribeAllAsync, PerPlatformProbeWatcher) opens
BeginPendingSubscribe() before the RPC and closes it after registering.
Inside the window unknown-handle events are parked (bounded, 100k); the
Register/Rebind that binds a handle promotes them, and the EventPump replays
them on its single dispatch thread before any later event for that handle
(woken by a sentinel when no further event arrives). Closing the last window
discards unclaimed events, so the stale drop is unchanged outside it.
New counters galaxy.events.early_{parked,replayed,overflow}.

Claude-Session: https://claude.ai/code/session_01GT67AD99zQQqpLCVm3p7xk

docs/v2/Galaxy.Performance.md

@@ -40,6 +40,9 @@ the metrics surface instead.
 | `galaxy.events.dropped` | `{event}` | MxEvents discarded because the bounded channel was full (newest-dropped) |
 | `galaxy.events.replay_gaps` | `{gap}` | Replay-gap sentinels received — the gateway evicted events we can never see |
 | `galaxy.events.undecodable` | `{event}` | Data-change events carrying a value kind this driver cannot represent |
+| `galaxy.events.early_parked` | `{event}` | Data-change events that overtook their SubscribeBulk reply and were parked for replay (#859) |
+| `galaxy.events.early_replayed` | `{event}` | Parked early events replayed once the reply registered their handle (#859) |
+| `galaxy.events.early_overflow` | `{event}` | Early events dropped because the parking bound (100 000) was reached (#859) |
 
 The invariant is `received = dispatched + dropped + replay_gaps +
 (in-flight in the channel)`. A replay-gap sentinel is counted as