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.AbCip/AbCipTemplateCache.cs

@@ -4,9 +4,13 @@ namespace ZB.MOM.WW.OtOpcUa.Driver.AbCip;
 
 /// <summary>
 ///     Cache of UDT shape descriptors keyed by <c>(deviceHostAddress, templateInstanceId)</c>.
-///     Populated on demand during discovery + whole-UDT reads; flushed via
-///     <see cref="AbCipDriver.FlushOptionalCachesAsync"/> and on device
-///     <c>ReinitializeAsync</c>.
+///     Populated on demand during discovery + whole-UDT reads.
+///     <para><b>Nothing in production clears it.</b> Its only caller was
+///     <c>AbCipDriver.FlushOptionalCachesAsync</c>, deleted in Gitea #525 as an unconsumed
+///     <c>IDriver</c> member — but that member never had a caller either, so this is not a change in
+///     behaviour, only in how visible the fact is. The comment here used to claim a device
+///     <c>ReinitializeAsync</c> also flushed it; no such call ever existed. A UDT redefined on a live
+///     controller therefore keeps its stale shape until the driver instance is rebuilt.</para>
 /// </summary>
 /// <remarks>
 ///     Templates are decoded by <see cref="CipTemplateObjectDecoder"/> (CIP Template Object
@@ -34,7 +38,8 @@ public sealed class AbCipTemplateCache
     public void Put(string deviceHostAddress, uint templateInstanceId, AbCipUdtShape shape) =>
         _shapes[(deviceHostAddress, templateInstanceId)] = shape;
 
-    /// <summary>Drop every cached shape — called on <see cref="AbCipDriver.FlushOptionalCachesAsync"/>.</summary>
+    /// <summary>Drop every cached shape. Retained as ordinary cache API + the tests' reset seam;
+    /// see the type remarks for why no production path calls it.</summary>
     public void Clear() => _shapes.Clear();
 
     /// <summary>Count of cached shapes — exposed for diagnostics + tests.</summary>