fix(dcl+centralui): MxGateway tag browse — lazy attributes, frame-size cap, wider scrollable picker

Expanding a Galaxy object in the tag picker hung on "loading…": the browse
reply inlined every child's full attribute set (~152 KB), exceeding Akka's
128 KB remote frame, and remoting silently discarded the oversized reply.

Browse path (DataConnectionLayer):
- RealMxGatewayClient: navigation now uses BrowseChildren(include_attributes=
  false) — child objects only — and an object's own attributes load lazily via
  DiscoverHierarchy(root, max_depth=0) when it's expanded. Payload drops from
  ~152 KB/level to a few KB. Seam contract unchanged.
- DataConnectionActor.CapBrowseChildren: protocol-agnostic byte-budget cap
  (~100 KB) on every BrowseNodeResult before it crosses the site→central
  frame, OR-ing the adapter's own Truncated flag. Byte budget, not a count —
  the only bound that holds regardless of NodeId/attribute-name length.
- RealOpcUaClient: requestedMaxReferencesPerNode 1000 → 500 to narrow the
  window before the byte budget applies.
- Graceful gRPC Unimplemented handling → NotSupportedException →
  BrowseFailureKind.NotBrowsable with an actionable message (older gateway
  builds lacking BrowseChildren).

Picker UI (CentralUI):
- NodeBrowserDialog: modal-lg → modal-xl; new scoped .razor.css caps the tree
  at 55vh with its own scrollbar so manual entry + Select/Cancel stay visible.
- Protocol-agnostic failure messages (was hardcoded "OPC UA …"); renamed the
  leftover opcua-browser-tree class to node-browser-tree.

Tests: new frame-budget cap test + NotSupported=>NotBrowsable mapping test;
DCL suite 88/88. Doc: Component-DataConnectionLayer.md records the lazy
attribute-light browse and the frame-size guard.

docs/requirements/Component-DataConnectionLayer.md

@@ -62,7 +62,7 @@ All protocols produce the same value tuple consumed by Instance Actors. Before t
 - Connects to the **MxAccess Gateway** (AVEVA/Wonderware MXAccess-backed Galaxy) over gRPC using the `ZB.MOM.WW.MxGateway.Client` NuGet package (from the Gitea feed); `ZB.MOM.WW.MxGateway.Contracts` is pulled in transitively.
 - Session-based: `OpenSession` + `Register` on connect; `AddItem` + `Advise` per subscription; value changes arrive on the gateway's server-streaming event feed (`StreamEvents`), resumable via `worker_sequence`.
 - Read/Write via `ReadBulk` / `WriteBulk`; writes carry a configurable `WriteUserId`. Quality maps the OPC-style quality byte (≥192 Good, ≥64 Uncertain, else Bad), with a failing MXAccess status proxy treated as Bad.
-- Galaxy hierarchy browse via the separate `GalaxyRepositoryClient` (`BrowseChildren`) — objects are navigable nodes (keyed by Galaxy gobject id), attributes are selectable leaves (keyed by full tag reference).
+- Galaxy hierarchy browse via the separate `GalaxyRepositoryClient` — objects are navigable nodes (keyed by Galaxy gobject id), attributes are selectable leaves (keyed by full tag reference). Browse is **lazy and attribute-light**: navigation uses `BrowseChildren` with `include_attributes=false` (child objects only), and an object's own attributes are fetched only when it is expanded, via `DiscoverHierarchy(root=<object>, max_depth=0)` scoped to that single object. This keeps each browse level's reply small; inlining every child's full attribute set could exceed the Akka remote frame and silently drop the reply.
 - Disconnect detection: a fault on the event stream raises `IDataConnection.Disconnected`, driving the same reconnection state machine as OPC UA.
 - Implemented as `MxGatewayDataConnection` over an `IMxGatewayClient` seam; the seam is decoupled from the generated gRPC types (only `RealMxGatewayClient` references them), so the adapter is fully unit-testable with a fake.
 
@@ -171,6 +171,7 @@ DCL is a clean data pipe on the hot path. Browse is an **opt-in capability** for
 - `DataConnectionManagerActor` handles `BrowseNodeCommand` (fields: `ConnectionName`, `ParentNodeId`) and replies with `BrowseNodeResult` (children + `Truncated` + structured `BrowseFailure?`). The Central UI facade is `IBrowseService`/`BrowseService`, backing the `NodeBrowserDialog` tag picker.
 - Node ids are opaque protocol-specific strings: OPC UA uses NodeIds; MxGateway uses Galaxy gobject ids for navigable objects and full tag references for selectable attribute leaves.
 - Browse runs against the live session; no caching at DCL.
+- **Frame-size guard**: the reply crosses the site→central Akka frame (default 128 KB) on a temp Ask actor; an oversized reply is silently discarded by remoting, hanging the picker. The child handler caps each `BrowseNodeResult` to a byte budget (~100 KB) before replying, OR-ing the adapter's own truncation signal into `Truncated`. This is protocol-agnostic (every adapter's reply funnels through it). Per-protocol upstream caps narrow the window first: OPC UA requests at most 500 references per node (continuation point → `Truncated`); MxGateway relies on the gateway's `BrowseChildren` page cap. A `Truncated` level prompts manual node-id entry in the picker rather than auto-paging.
 
 ## Value Update Message Format