refactor(drivers): delete GetMemoryFootprint / FlushOptionalCachesAsync (#525)

Both `IDriver` members had no production call site. Their only consumer was
`MemoryTracking`, deleted with the driver-tier recycle machinery in #522, and
`GetMemoryFootprint`'s doc-comment claimed "polled every 30s by Core" — which
had not been true since v3, even before #522 removed the poller.

Owner decision on #525 was (a) delete rather than (b) build the process-wide
watchdog that would consume them. The driver-side implementations are in git
history and were mostly reusable; what was always missing is the trigger, and a
member nobody calls is how the misleading doc-comment survived this long.

Removed from `IDriver`, all 12 driver implementations, and every test stub —
plus the support code that existed only to feed them, which is the part a
member-only deletion would have left behind as dead weight:

- MTConnect's three per-entry byte-weight constants, its cached
  `ProbeModelDataItemCount`, and `CountDataItems` — a full device-tree walk run
  on every cache fill to populate a number with no remaining reader.
- OpcUaClient's `_discoveredNodeCount` field and the discovery-pass write to it.
- MQTT's `ApproxBytesPerAuthoredTag`.

Two behavioural notes, neither a change in what the software does:

- `AbCipTemplateCache` now has no production reset path. Its only caller was
  AB CIP's flush, which itself had no caller — so nothing changes except how
  visible the fact is. Recorded on the type, along with the correction that its
  comment's claim about a device `ReinitializeAsync` also flushing it was never
  true. A UDT redefined on a live controller keeps its stale shape until the
  driver instance is rebuilt.
- MTConnect's `/probe` cache is still dropped, by `InvalidateProbeModel()` on an
  Agent restart. That is now `internal` rather than `private` so the tests which
  used the flush purely to empty the cache drive the real path instead — they
  now cover a case that actually occurs in production. Several unrelated doc
  comments cited `FlushOptionalCachesAsync` as the canonical CAS example and are
  repointed to it.

Tests: the ones that existed only to assert on the removed members are deleted
(3 OpcUaClient, 3 Galaxy, 2 MQTT, 1 MTConnect, 1 AbCip). Worth noting one was
vacuous — `FlushOptionalCachesAsync_resets_footprint_counter` asserted a
footprint of 0 before the flush and 0 after with no discovery in between, so it
would have passed against a flush that did nothing at all.

Full suite green except 7 failures verified identical on master before and
after (4 AbLegacy integration with the compose profile down, 3
Host.IntegrationTests), checked by name rather than assumed.

Docs corrected where they described this as live: `DriverLifecycle.md` (which
carried the #525 placeholder), `drivers/AbCip.md`, `drivers/AbLegacy.md`, and
`v2/driver-stability.md` — whose "polls this every 30 s" + per-driver
`DriverConfig` memory budget were never built as written and are now banner-ed
as design intent rather than description.

Claude-Session: https://claude.ai/code/session_015p7wGqy3YpZNCpDzTpGMKo

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Mqtt/MqttDriver.cs

@@ -88,15 +88,6 @@ public sealed class MqttDriver
     /// <summary>The publish failed, or the driver has no live broker connection to publish through.</summary>
     private const uint StatusBadCommunicationError = 0x80050000u;
 
-    /// <summary>
-    ///     Rough per-tag driver-attributable cost: the parsed <see cref="MqttTagDefinition"/> (five
-    ///     strings — RawPath, topic, JSONPath and the record header) plus its cached
-    ///     <see cref="DataValueSnapshot"/> and the two dictionary entries that index it. An estimate
-    ///     by construction — <c>GetMemoryFootprint</c> exists to drive a cache-budget decision, not
-    ///     to be exact.
-    /// </summary>
-    private const long ApproxBytesPerAuthoredTag = 512;
-
     /// <summary>
     ///     The single folder <see cref="DiscoverAsync"/> streams every variable under — and therefore
     ///     the only subtree path a rediscovery <c>ScopeHint</c> can name truthfully. See
@@ -429,24 +420,6 @@ public sealed class MqttDriver
         };
     }
 
-    /// <summary>
-    ///     Approximate driver-attributable footprint: the authored definition table and the
-    ///     last-value cache that indexes it. Both are sized by the authored tag count, which is the
-    ///     only thing a deployment can grow.
-    /// </summary>
-    /// <returns>The approximate memory footprint in bytes.</returns>
-    public long GetMemoryFootprint() => _authoredRawPaths.Count * ApproxBytesPerAuthoredTag;
-
-    /// <summary>
-    ///     No-op, deliberately. This driver holds no optional cache: the authored table is the
-    ///     address space itself, and the last-value cache <b>is</b> <see cref="IReadable"/> — MQTT is
-    ///     subscribe-first, so a dropped value is not re-fetchable, and flushing it would turn every
-    ///     node Bad until its topic next published, which for a slow-changing tag can be never.
-    ///     Birth/alias state (Sparkplug, P2) is correctness state for the same reason.
-    /// </summary>
-    /// <param name="cancellationToken">Unused; present for the <see cref="IDriver"/> shape.</param>
-    /// <returns>A completed task.</returns>
-    public Task FlushOptionalCachesAsync(CancellationToken cancellationToken) => Task.CompletedTask;
 
     // ---- ITagDiscovery ----