docs: dashboard live-data session reaping, subscriber-refusal code, distributor-based StreamEvents model

- Sessions.md / GatewayConfiguration.md: the dashboard Browse live-data session is
  unary-only, is reaped by UnattachedSessionGraceSeconds when idle, and reopens via
  EnsureReadyAsync; only the alarm monitor is exempt.
- GatewayConfiguration.md: a refused second subscriber gets ResourceExhausted, not AlreadyExists.
- Grpc.md: rewrite the event streaming model around the SessionEventDistributor pump and
  replay ring; add EventSubscriberLimitReached to the mapping snippet; describe what
  clients see while the gateway is stopping.

docs/GatewayConfiguration.md

@@ -154,8 +154,8 @@ validation fails fast when either half of the headroom rule is violated.
 | `MxGateway:Sessions:LeaseSweepIntervalSeconds` | `30` | Hosted monitor interval for closing expired leases. Active event-stream subscribers keep a session from expiring while the stream remains attached. |
 | `MxGateway:Sessions:DetachGraceSeconds` | `30` | Detach-grace retention window. When positive, a session whose last external (gRPC) event-stream subscriber drops is retained in `Ready` for this many seconds so a client can reconnect; if no external subscriber re-attaches within the window, the lease monitor closes it with `detach-grace-expired`. The internal dashboard mirror does not count as an external subscriber, so a dashboard-only session still enters detach-grace. `0` disables retention and reverts to closing only on normal lease expiry. Must be zero or greater. Reconnect/replay itself is implemented separately (Task 12); this option controls retention and expiry only. The effective close happens within the next sweep cycle after the window elapses — up to `LeaseSweepIntervalSeconds` after expiry. Operators wanting a firm minimum retention bound should set `DetachGraceSeconds` greater than `LeaseSweepIntervalSeconds`. |
 | `MxGateway:Sessions:FaultedGraceSeconds` | `0` | Grace window before the lease monitor reaps a faulted session (killing its worker and freeing the slot). A faulted session is permanently unusable yet otherwise pins a session slot and a live x86 worker until its normal lease expires. `0` (the default) reaps it on the next sweep cycle, bounding blast radius; a positive value keeps the faulted session observable via `GetSessionStatus` for that window before it is reclaimed, with the close reason `faulted-reaped`. Must be zero or greater. |
-| `MxGateway:Sessions:UnattachedSessionGraceSeconds` | `0` | Idle window after which the lease monitor reaps a `Ready` session that has **never** attached a `StreamEvents` subscriber, with the close reason `unattached-grace-expired`. Exists because a client that opens a session and vanishes before it ever streams — OtOpcUa's `GalaxyDriver` failing part-way through start-up, for example — otherwise pins a session slot and a live x86 worker for the full `DefaultLeaseSeconds` (30 minutes). The window is measured from the later of the session becoming `Ready` and its last unary command, so a slow worker start does not eat it and a unary-only client that keeps calling is never reaped by this rule; it stops applying once the session's first subscriber attaches (`DetachGraceSeconds` and the normal lease govern it from then on), and the gateway's own alarm-monitor session, whose subscriber is internal, is exempt the same way. `0` (the default) disables it. Must be zero or greater. Set it comfortably above the time a well-behaved client needs between `OpenSession` and its first `StreamEvents`; the close lands on the next sweep after the window elapses, up to `LeaseSweepIntervalSeconds` later. |
-| `MxGateway:Sessions:AllowMultipleEventSubscribers` | `false` | Controls whether multiple `StreamEvents` subscribers may attach to one session. When `false` the session refuses a second subscriber with `AlreadyExists`. Set to `true` to enable fan-out via the `SessionEventDistributor`. |
+| `MxGateway:Sessions:UnattachedSessionGraceSeconds` | `0` | Idle window after which the lease monitor reaps a `Ready` session that has **never** attached a `StreamEvents` subscriber, with the close reason `unattached-grace-expired`. Exists because a client that opens a session and vanishes before it ever streams — OtOpcUa's `GalaxyDriver` failing part-way through start-up, for example — otherwise pins a session slot and a live x86 worker for the full `DefaultLeaseSeconds` (30 minutes). The window is measured from the later of the session becoming `Ready` and its last unary command, so a slow worker start does not eat it and a unary-only client that keeps calling is never reaped by this rule; it stops applying once the session's first subscriber attaches (`DetachGraceSeconds` and the normal lease govern it from then on), and the gateway's own alarm-monitor session, whose subscriber is internal, is exempt the same way. The dashboard Browse live-data session (`DashboardLiveDataService`) is not exempt: it is unary-only, so it is reaped once idle for the window, and it opens a new session through `EnsureReadyAsync` on its next use. `0` (the default) disables it. Must be zero or greater. Set it comfortably above the time a well-behaved client needs between `OpenSession` and its first `StreamEvents`; the close lands on the next sweep after the window elapses, up to `LeaseSweepIntervalSeconds` later. |
+| `MxGateway:Sessions:AllowMultipleEventSubscribers` | `false` | Controls whether multiple `StreamEvents` subscribers may attach to one session. When `false` the session refuses a second subscriber with `ResourceExhausted` (`EventSubscriberAlreadyActive`). Set to `true` to enable fan-out via the `SessionEventDistributor`. |
 | `MxGateway:Sessions:MaxEventSubscribersPerSession` | `8` | Maximum number of concurrent `StreamEvents` subscribers per session when `AllowMultipleEventSubscribers` is `true`. Effectively 1 when `AllowMultipleEventSubscribers` is `false`. Must be greater than zero. |
 | `MxGateway:Sessions:WorkerReadyWaitTimeoutMs` | `0` | Bounded time, in milliseconds, the gateway will wait for a worker to reach `Ready` when the session is already `Ready` but the worker state has transiently diverged (e.g. `Handshaking` after a heartbeat blip). Applies only to transient worker states; terminal states (`Faulted`/`Closing`/`Closed`/no worker) fail fast immediately regardless of this setting. `0` (the default) disables the wait and preserves the original fail-fast behavior. Must be greater than or equal to zero. |