Phase 6.2 Stream C — CreateMonitoredItems per-item gating

Closes task #121 (partial — creation-time gate; decision #153 per-item
revocation stamp is a follow-up).

Before this commit a session could subscribe to any node via
CreateMonitoredItems, even nodes where Read was denied — the
subscription would surface BadUserAccessDenied on each data-change
read, but the client saw a successful CreateMonitoredItems response
and held the subscription open, wasting resources and leaking the
address-space shape through the item metadata.

New override on DriverNodeManager.CreateMonitoredItems:
- Pre-iterates itemsToCreate, gates each through AuthorizationGate with
  OpcUaOperation.CreateMonitoredItems at the target node's scope.
- For denied slots: sets errors[i] = new ServiceResult(
  StatusCodes.BadUserAccessDenied). The OPC Foundation base stack
  honours pre-populated non-success errors and skips item creation for
  those slots — the subscription never holds a handle to a denied
  node.
- Preserves prior errors (e.g. BadNodeIdUnknown) — first diagnosis wins.
- Non-string-identifier references (stack-synthesized numeric ids)
  bypass the gate.

Extracted the pure filter logic into
GateMonitoredItemCreateRequests(items, errors, identity, gate,
scopeResolver) — static internal, unit-testable without the OPC UA
server stack.

Tests — 6 new in MonitoredItemGatingTests.cs (gate-null no-op,
denied-gets-BadUserAccessDenied, allowed-passes, mixed-batch-denies-
per-item, pre-populated-error-preserved, numeric-id-bypass). Server.Tests
263 → 269.

Known follow-ups:
- Per-item (AuthGenerationId, MembershipVersion) stamp (decision #153)
  for detecting revocation mid-subscription — needs subscription-layer
  plumbing.
- TransferSubscriptions not yet wired (same pattern, smaller scope).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/v2/v2-release-readiness.md

@@ -35,7 +35,7 @@ All code-path release blockers are closed. The remaining items are live-hardware
 Remaining Stream C surfaces (hardening, not release-blocking):
 
 - ~~Browse + TranslateBrowsePathsToNodeIds gating with ancestor-visibility logic per `acl-design.md` §Browse.~~ **Partial, 2026-04-24.** `DriverNodeManager.Browse` override post-filters the `ReferenceDescription` list via a new `FilterBrowseReferences` helper — denied nodes disappear silently per OPC UA convention. Ancestor-visibility implication (Read-grant at `Line/Tag` implying Browse on `Line`) still to ship; needs a subtree-has-any-grant query on the trie evaluator. `TranslateBrowsePathsToNodeIds` surface not yet wired.
-- CreateMonitoredItems + TransferSubscriptions gating with per-item `(AuthGenerationId, MembershipVersion)` stamp so revoked grants surface `BadUserAccessDenied` within one publish cycle (decision #153).
+- ~~CreateMonitoredItems + TransferSubscriptions gating with per-item `(AuthGenerationId, MembershipVersion)` stamp so revoked grants surface `BadUserAccessDenied` within one publish cycle (decision #153).~~ **Partial, 2026-04-24.** `DriverNodeManager.CreateMonitoredItems` override pre-gates each request and pre-populates `BadUserAccessDenied` into the errors slot for denied items (the base stack honours pre-set errors and skips those items). Decision #153's per-item `(AuthGenerationId, MembershipVersion)` stamp for detecting mid-subscription revocation is still to ship — needs subscription-layer plumbing. TransferSubscriptions not yet wired (same pattern).
 - Alarm Acknowledge / Confirm / Shelve gating.
 - Call (method invocation) gating.
 - Finer-grained scope resolution — current `NodeScopeResolver` returns a flat cluster-level scope. Joining against the live Configuration DB to populate UnsArea / UnsLine / Equipment path is tracked as Stream C.12.