docs(drivers): un-stale the MQTT README rows + track every known gap as an issue

The cross-driver README still described MQTT as pre-P2 — "Sparkplug B is P2
(`mode: SparkplugB` is a placeholder)" and "Sparkplug B has no fixture yet (P2)".
Both were false as of the P2 merge (c3a2b0f7): SparkplugB is a fully implemented
mode and the live suite is 15 tests (7 plain + 8 Sparkplug) against a
project-owned C# edge-node simulator. Mqtt.md and the scadaproj index were both
already correct; only this file missed the P2 sweep.

Filed the nine Known-gaps rows as tracking issues and linked them, so the table
stops being the only record of them:

  #507  IRediscoverable is inert in v3 (platform gap — also Galaxy/TwinCAT/AbCip)
  #508  write-through (IWritable) — NCMD/DCMD + plain publish
  #509  a metric name containing '/' cannot be browse-committed
  #510  _canRebirth captured at browse-open — reaped session still draws button
  #511  Sparkplug primary-host STATE publishing (ActAsPrimaryHost)
  #512  .Browser references .Driver — extract the client-options builder
  #513  editor writes a meaningless payloadFormat into a Sparkplug blob
  #514  Request-rebirth unarmable on a plant that birthed before the window
  #515  DataSet / Template / PropertySet metric support

#507 is the one worth reading: it is a v3 platform gap, not an MQTT bug —
DriverHostActor.HandleDiscoveredNodes hard-returns, so every IRediscoverable
driver fires into a void and the edge gaining a point needs a redeploy to show up.

No code change; docs + issue links only.

docs/drivers/README.md

@@ -29,7 +29,7 @@ Driver type metadata is registered at startup in `DriverTypeRegistry` (`src/Core
 | [TwinCAT](TwinCAT.md) | `Driver.TwinCAT` | B | Beckhoff `TwinCAT.Ads` (`TcAdsClient`) | IDriver, ITagDiscovery, IReadable, IWritable, ISubscribable, IHostConnectivityProbe, IPerCallHostResolver, IRediscoverable | The only native-notification driver outside Galaxy — ADS delivers `ValueChangedCallback` events the driver forwards straight to `ISubscribable.OnDataChange` without polling. Symbol tree uploaded via `SymbolLoaderFactory` |
 | [FOCAS](FOCAS.md) | `Driver.FOCAS` | A | Pure-managed `FocasWireClient` — FOCAS/2 Ethernet binary protocol on TCP:8193, inlined into the driver assembly | IDriver, ITagDiscovery, IReadable, IWritable, ISubscribable, IHostConnectivityProbe, IPerCallHostResolver, IAlarmSource | `IWritable` is implemented but read-only by design — `WriteAsync` returns `BadNotWritable` for every point. CNC-shaped data model (axes, spindle, PMC, macros, alarms) not a flat tag map. Previously Tier-C (Host + P/Invoke + shim DLL); retired in the 2026-04-24 migration when the managed wire client landed |
 | [OPC UA Client](OpcUaClient.md) | `Driver.OpcUaClient` | B | OPCFoundation `Opc.Ua.Client` | IDriver, ITagDiscovery, IReadable, IWritable, ISubscribable, IAlarmSource, IHistoryProvider, IHostConnectivityProbe | Gateway/aggregation driver — the only driver implementing driver-side `IHistoryProvider` (forwards HistoryRead to the upstream server). Opens a single `Session` against a remote OPC UA server and re-exposes its address space. Owns its own `ApplicationConfiguration` (distinct from `Client.Shared`) because it's always-on with keep-alive + `TransferSubscriptions` across SDK reconnect, not an interactive CLI |
-| [MQTT](Mqtt.md) | `Driver.Mqtt` (+ `.Browser`, `.Contracts`) | A | [MQTTnet 5](https://github.com/dotnet/MQTTnet) | IDriver, ITagDiscovery, IReadable, ISubscribable, IHostConnectivityProbe, IRediscoverable | **Push, not poll, and read-only** — the broker delivers PUBLISH frames the driver forwards straight to `ISubscribable.OnDataChange`; `IReadable` serves the last retained/received value from cache, and there is **no `IWritable`** (nothing publishes back). Subscriptions are indexed **by topic filter, not by topic**, so wildcard tags survive a reconnect. A rejected CONNACK throws `MqttConnectRejectedException`: unrecoverable codes → `Faulted` + supervisor stop, transient → retry. Sparkplug B is P2 (`mode: SparkplugB` is a placeholder) |
+| [MQTT](Mqtt.md) | `Driver.Mqtt` (+ `.Browser`, `.Contracts`) | A | [MQTTnet 5](https://github.com/dotnet/MQTTnet) | IDriver, ITagDiscovery, IReadable, ISubscribable, IHostConnectivityProbe, IRediscoverable | **Push, not poll, and read-only** — the broker delivers PUBLISH frames the driver forwards straight to `ISubscribable.OnDataChange`; `IReadable` serves the last retained/received value from cache, and there is **no `IWritable`** (nothing publishes back). Subscriptions are indexed **by topic filter, not by topic**, so wildcard tags survive a reconnect. A rejected CONNACK throws `MqttConnectRejectedException`: unrecoverable codes → `Faulted` + supervisor stop, transient → retry. **Two modes:** `Plain` binds a tag to a concrete topic + JSON path; `SparkplugB` decodes vendored Eclipse-Tahu protobuf under one `spBv1.0/{groupId}/#` subscription and binds by the `group/edgeNode/device?/metric` tuple (birth/alias/seq-gap state machine, death→STALE, scoped rebirth NCMD). `IRediscoverable` fires on a DBIRTH but is **inert platform-side** — see [#507](https://gitea.dohertylan.com/dohertj2/lmxopcua/issues/507) |
 | [Historian.Gateway](../Historian.md) | `Driver.Historian.Gateway` | — | `ZB.MOM.WW.HistorianGateway.Client` gRPC (`historian_gateway.v1`) | IHistorianDataSource (server-side read backend) + alarm `SendEvent` writer + `WriteLiveValues` recorder + `IHistorianProvisioning` | Not a tag driver — the sole historian backend. Registers `GatewayHistorianDataSource : IHistorianDataSource` for HistoryRead and serves alarm-write + continuous historization through the gateway. No `IDriver`/`ITagDiscovery` surface. (The retired Wonderware sidecar backend it replaced is documented at [Historian.Wonderware.md](Historian.Wonderware.md).) |
 
 ## Per-driver documentation
@@ -48,7 +48,7 @@ Driver type metadata is registered at startup in `DriverTypeRegistry` (`src/Core
   - [S7.md](S7.md) — Siemens S7 driver (S7-300/400/1200/1500 + S7-200): getting started, config, data-block addressing, serialized single-connection model
   - [TwinCAT.md](TwinCAT.md) — Beckhoff TwinCAT (ADS) driver: getting started, native-notification subscription, symbol-tree upload
   - [OpcUaClient.md](OpcUaClient.md) — OPC UA Client (gateway/aggregation) driver: remote-server session, driver-side HistoryRead forwarding, reconnect behaviour
-  - [Mqtt.md](Mqtt.md) — MQTT broker-subscribe driver: broker/TLS/auth config, topic + JSON-path tag binding, retained-value seeding, `#`-observation browser, CONNACK-rejection semantics
+  - [Mqtt.md](Mqtt.md) — MQTT broker-subscribe driver: broker/TLS/auth config, topic + JSON-path tag binding, retained-value seeding, `#`-observation browser, CONNACK-rejection semantics, **Sparkplug B** (birth/alias/seq/rebirth state machine, birth-driven browse picker, death→STALE), and the **Known gaps** table
 
 - **Historian.Gateway** (server-side historian backend, not a tag driver) is documented in the main guide:
   - [../Historian.md](../Historian.md) — HistorianGateway backend: read-path registration, HistoryRead dispatch, alarm store-and-forward (`SendEvent`), continuous historization (`WriteLiveValues`), `EnsureTags` provisioning, config keys, deployment prerequisites. (The retired Wonderware sidecar backend it replaced: [Historian.Wonderware.md](Historian.Wonderware.md).)
@@ -66,7 +66,7 @@ Each driver has a dedicated fixture doc that lays out what the integration / uni
 - [TwinCAT](TwinCAT-Test-Fixture.md) — XAR-VM integration scaffolding (task #221); three smoke tests skip when VM unreachable. Unit via `FakeTwinCATClient` with native-notification harness
 - [FOCAS](FOCAS-Test-Fixture.md) — no integration fixture, unit-only via `FakeFocasClient`; Tier C out-of-process isolation scoped but not shipped
 - [OPC UA Client](OpcUaClient-Test-Fixture.md) — Dockerized `opc-plc` integration suite (task #215): real Secure Channel + Session, read + subscribe verified end-to-end; write not yet exercised in the integration suite; exhaustive capability matrix (reconnect, failover, cert-auth, history, alarms) via unit suite with mocked `Session`
-- [MQTT](Mqtt-Test-Fixture.md) — Dockerized `eclipse-mosquitto` with **TLS + real auth on both listeners (no anonymous fallback)** + a JSON-publisher sidecar; env-gated live suite (7 tests) covers TLS/auth connect, CA pinning both ways, subscribe→value, retained seeding, and wrong-password rejection. Sparkplug B has no fixture yet (P2)
+- [MQTT](Mqtt-Test-Fixture.md) — Dockerized `eclipse-mosquitto` with **TLS + real auth on both listeners (no anonymous fallback)** + a JSON-publisher sidecar; env-gated live suite (**15 tests**) — 7 plain (TLS/auth connect, CA pinning both ways, subscribe→value, retained seeding, wrong-password rejection) + 8 Sparkplug against a **project-owned C# edge-node simulator** on the `sparkplug` compose profile (birth→alias bind, DDATA, NDEATH→STALE, rebirth NCMD round-trip). Births are never retained, so `docker restart otopcua-sparkplug-sim` is how you force a fresh one
 - [Galaxy](../v1/drivers/Galaxy-Test-Fixture.md) — richest harness: gateway E2E + ZB SQL live-smoke + MXAccess opt-in (v1 archive)
 
 ## Related cross-driver docs