Phase 6.2 Stream C — Browse gating on DriverNodeManager

Closes task #120 (partial — strict point-check; ancestor-visibility
implication is a follow-up).

Before this commit DriverNodeManager exposed every materialized node to
every browsing session regardless of the user's ACL. Read + Write +
HistoryRead were already gated through AuthorizationGate in Phase 6.2
Stream C core; Browse was the one surface where the session could still
enumerate nodes it had no permission to touch, discovering structure
even when reads failed with BadUserAccessDenied.

Implementation
- New `Browse` override on DriverNodeManager that calls base.Browse
  first (lets the stack populate the reference list normally), then
  post-filters the IList<ReferenceDescription> so denied nodes are
  removed silently. OPC UA convention: Browse filtering is invisible to
  the client; no BadUserAccessDenied surfaces.
- Extracted the filter loop into the static internal
  `FilterBrowseReferences(references, userIdentity, gate, scopeResolver)`
  so the policy is unit-testable without standing up the full OPC UA
  server stack.
- Non-string NodeId identifiers (stack-synthesized standard-type
  references with numeric identifiers) bypass the gate — only driver-
  materialized nodes key into the authz trie.
- When AuthorizationGate or NodeScopeResolver is null, the filter is a
  no-op — preserves the pre-Phase-6.2 dispatch path for integration
  tests that construct DriverNodeManager without authz.

Tests — 6 new in BrowseGatingTests.cs (gate-null no-op, empty-list
no-op, denied-removed, allowed-passes-through, numeric-id bypass,
lax-mode null-identity keeps references). Server.Tests 257 → 263.

Known follow-up (tracked implicitly under #120 re-scope):
- Ancestor-visibility implication (acl-design.md §Browse line 111): a
  user with Read at `Line/Tag` should be able to Browse `Line` even
  without an explicit Browse grant. Current filter does a strict
  point-check. Proper fix needs TriePermissionEvaluator to expose a
  "subtree-has-any-grant" query.
- TranslateBrowsePathsToNodeIds not yet filtered (same extension
  pattern; small follow-up).

Docs: v2-release-readiness.md Phase 6.2 Stream C hardening list marks
the Browse bullet struck-through with "Partial" close-out note.

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

docs/v2/v2-release-readiness.md

@@ -34,7 +34,7 @@ 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.
+- ~~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).
 - Alarm Acknowledge / Confirm / Shelve gating.
 - Call (method invocation) gating.