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.OpcUaClient/OpcUaClientDriver.cs

@@ -129,12 +129,6 @@ public sealed class OpcUaClientDriver : IDriver, ITagDiscovery, IReadable, IWrit
     /// <summary>The <see cref="CertificateValidator"/> that owns <see cref="_certValidationHandler"/>.</summary>
     private CertificateValidator? _certValidatorRef;
     /// <summary>
-    ///     Approximate count of discovered nodes (folders + variables). Updated by
-    ///     <see cref="DiscoverAsync"/> and used to report a non-zero
-    ///     <see cref="GetMemoryFootprint"/> to the Core allocation-slope detector.
-    /// </summary>
-    private volatile int _discoveredNodeCount;
-    /// <summary>
     ///     SDK-provided reconnect handler that owns the retry loop + session-transfer machinery
     ///     when the session's keep-alive channel reports a bad status. Null outside the
     ///     reconnecting window; constructed lazily inside the keep-alive handler. Guarded by
@@ -644,23 +638,6 @@ public sealed class OpcUaClientDriver : IDriver, ITagDiscovery, IReadable, IWrit
     /// <inheritdoc />
     public DriverHealth GetHealth() => _health;
 
-    /// <inheritdoc />
-    // Each discovered node (folder or variable) contributes ~512 bytes to cover the
-    // DriverAttributeInfo record, the browse-name string, and the stable nsu= reference string
-    // stored in the address-space builder. The real number depends on string length + box sizes;
-    // the constant is conservative enough that a 10k-node remote server reports ~5 MB — well
-    // within the budget and detectable by the Core slope alarm.
-    public long GetMemoryFootprint() => _discoveredNodeCount * 512L;
-
-    /// <inheritdoc />
-    // The OPC UA Client driver holds no independently-flushable cache beyond what the
-    // address-space builder retains — a flush here resets the discovered-node footprint
-    // counter and signals Core that re-discovery will rebuild it cleanly from the remote server.
-    public Task FlushOptionalCachesAsync(CancellationToken cancellationToken)
-    {
-        _discoveredNodeCount = 0;
-        return Task.CompletedTask;
-    }
 
     // ---- IReadable ----
 
@@ -926,9 +903,6 @@ public sealed class OpcUaClientDriver : IDriver, ITagDiscovery, IReadable, IWrit
             await EnrichAndRegisterVariablesAsync(session, pendingVariables, cancellationToken)
                 .ConfigureAwait(false);
 
-            // Update the footprint counter so GetMemoryFootprint() returns a real estimate
-            // after each discovery pass.
-            _discoveredNodeCount = discovered;
         }
         finally { _gate.Release(); }
     }