R4.3: measured idle-state GetStoreForwardStatusAsync over gRPC

Route GetStoreForwardStatusAsync to a gRPC path that actually contacts the
server (StatusService.GetHistorianConsoleStatus) instead of synthesizing an
all-false result blind. On a reachable/normal server it returns the
not-storing baseline but MEASURED; when the server is unreachable or the
console-status call fails it reports ErrorOccurred with the underlying error
(the old synthesis never contacted the server). The active-SF buffer
magnitude (Storing/Pending/DataStored) stays false because it lives behind
the D2 storage-engine console wall.

Non-gRPC transports keep the synthesized fallback. Live-verified against the
2023 R2 server; gated integration test
GetStoreForwardStatusAsync_OverGrpc_ReturnsMeasuredIdleState added. README
operation table updated.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01B6mcaT2PjRFKcogzp9UkfC

README.md

@@ -17,7 +17,7 @@ The supported surface (per [`CLAUDE.md`](CLAUDE.md)):
 | `BrowseTagNamesAsync` | live-verified |
 | `GetTagMetadataAsync` | live-verified for 17 distinct native data-type codes |
 | `GetConnectionStatusAsync` | synthesized from authenticated probe (matches native semantic) |
-| `GetStoreForwardStatusAsync` | synthesized defaults (no SF sidecar to probe) |
+| `GetStoreForwardStatusAsync` | gRPC: measured idle-state (live-verified — contacts server, reports `ErrorOccurred` when unreachable; active-SF magnitude is D2-gated). Non-gRPC: synthesized defaults |
 | `GetSystemParameterAsync` | live-verified via `Stat/GetSystemParameter` |
 | `EnsureTagAsync` | live-verified for analog Float/Double/Int2/Int4/UInt4; `ApplyScaling=true` persists distinct MinRaw/MaxRaw |
 | `DeleteTagAsync` | live-verified |