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

docs/DriverLifecycle.md

@@ -136,15 +136,31 @@ the real per-capability timeout / retry / breaker policies, resolved via
 Every driver runs Tier A because no factory passes a tier, so today that means
 one uniform policy set.
 
-⚠️ Two `IDriver` members are now **consumerless**: `GetMemoryFootprint()` and
-`FlushOptionalCachesAsync()` existed to feed `MemoryTracking`. All 12 drivers
-still implement them and nothing calls them. They were left in place rather
-than swept, since removing them touches every driver and every test stub —
-tracked as Gitea **#525**.
+`IDriver` **no longer carries `GetMemoryFootprint()` or
+`FlushOptionalCachesAsync()`** (Gitea **#525**). They existed to feed
+`MemoryTracking`, deleted with the rest of the driver-tier recycle machinery in
+**#522**; after that nothing called them, and their doc-comments still claimed
+Core polled the footprint every 30 s, which had not been true since v3. The
+sweep removed them from the interface, all 12 drivers, and every test stub,
+along with the support code that existed only to feed them — MTConnect's
+per-entry byte weights and its cached data-item count, OpcUaClient's
+`_discoveredNodeCount`, and MQTT's per-tag byte estimate.
+
+Two consequences worth knowing:
+
+- `AbCipTemplateCache` now has **no production reset path**. Its only caller was
+  AB CIP's flush — which itself had no caller, so nothing has actually changed
+  in behaviour, only in how visible the fact is. 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. The tests that used to reach it through the flush now drive
+  that path instead, so they cover a case that actually occurs.
 
 If per-driver memory protection is wanted again, build it for the architecture
-as it is now: a process-wide watchdog, not a per-driver tier whose only remedy
-requires a supervisor that cannot exist in-process.
+as it is now: a process-wide watchdog that can ask drivers to flush, not a
+per-driver tier whose only remedy requires a supervisor that cannot exist
+in-process. The deleted per-driver implementations are in git history and were
+mostly reusable — what was always missing is the trigger.
 
 ---