Compute OIDC auth, async Horde agents, and orchestrator improvements (#913)

Rework of the Horde agent subsystem from synchronous per-thread I/O to an async ASIO-driven architecture, plus provisioner scale-down with graceful draining, OIDC authentication, scheduler improvements, and dashboard UI for provisioner control.

### Async Horde Agent Rewrite
- Replace synchronous `HordeAgent` (one thread per agent, blocking I/O) with `AsyncHordeAgent` — an ASIO state machine running on a shared `io_context` thread pool
- Replace `TcpComputeTransport`/`AesComputeTransport` with `AsyncTcpComputeTransport`/`AsyncAesComputeTransport`
- Replace `AgentMessageChannel` with `AsyncAgentMessageChannel` using frame queuing and ASIO timers
- Delete `ComputeBuffer` and `ComputeChannel` ring-buffer classes (no longer needed)

### Provisioner Drain / Scale-Down
- `HordeProvisioner` can now drain agents when target core count is lowered: queries each agent's `/compute/session/status` for workload, selects candidates by largest-fit/lowest-workload, and sends `/compute/session/drain`
- Configurable `--horde-drain-grace-period` (default 300s) before force-kill
- Implement `IProvisionerStateProvider` interface to expose provisioner state to the orchestrator HTTP layer
- Forward `--coordinator-session`, `--provision-clean`, and `--provision-tracehost` through both Horde and Nomad provisioners to spawned workers

### OIDC Authentication
- `HordeClient` accepts an `AccessTokenProvider` (refreshable token function) as alternative to static `--horde-token`
- Wire up `OidcToken.exe` auto-discovery via `httpclientauth::CreateFromOidcTokenExecutable` with `--HordeUrl` mode
- New `--horde-oidctoken-exe-path` CLI option for explicit path override

### Orchestrator & Scheduler
- Orchestrator generates a session ID at startup; workers include `coordinator_session` in announcements so the orchestrator can reject stale-session workers
- New `Rejected` action state — when a remote runner declines at capacity, the action is rescheduled without retry count increment
- Reduce scheduler lock contention: snapshot pending actions under shared lock, sort/trim outside the lock
- Parallelize remote action submission across runners via `WorkerThreadPool` with slow-submit warnings
- New action field `FailureReason` populated by all runner types (exit codes, sandbox failures, exceptions)
- New endpoints: `session/drain`, `session/status`, `session/sunset`, `provisioner/status`, `provisioner/target`

### Remote Execution
- Eager-attach mode for `RemoteHttpRunner` — bundles all attachments upfront in a `CbPackage` for single-roundtrip submits
- Track in-flight submissions to prevent over-queuing
- Show remote runner hostname in `GetDisplayName()`
- `--announce-url` to override the endpoint announced to the coordinator (e.g. relay-visible address)

### Frontend Dashboard
- Delete standalone `compute.html` (925 lines) and `orchestrator.html` (669 lines), consolidated into JS page modules
- Add provisioner panel to orchestrator dashboard: target/active/estimated core counts, draining agent count
- Editable target-cores input with debounced POST to `/orch/provisioner/target`
- Per-agent provisioning status badges (active / draining / deallocated) in the agents table
- Active vs total CPU counts in agents summary row

### CLI
- New `zen compute record-start` / `record-stop` subcommands
- `zen exec` progress bar with submit and completion phases, atomic work counters, `--progress` mode (Pretty/Plain/Quiet)

### Other
- `DataDir` supports environment variable expansion
- Worker manifest validation checks for `worker.zcb` marker to detect incomplete cached directories
- Linux/Mac runners `nice(5)` child processes to avoid starving the main server
- `ComputeService::SetShutdownCallback` wired to `RequestExit` via `session/sunset`
- Curl HTTP client logs effective URL on failure
- `MachineInfo` carries `Pool` and `Mode` from Horde response
- Horde bundle creation includes `.pdb` on Windows

1 files changed, 27 insertions, 2 deletions

diff --git a/src/zencompute/orchestratorservice.cpp b/src/zencompute/orchestratorservice.cpp
index 9ea695305..aee8fa63a 100644
--- a/src/zencompute/orchestratorservice.cpp
+++ b/src/zencompute/orchestratorservice.cpp
@@ -31,7 +31,7 @@ OrchestratorService::~OrchestratorService()
 }
 
 CbObject
-OrchestratorService::GetWorkerList()
+OrchestratorService::GetWorkerList(const WorkerAnnotator& Annotate)
 {
 	ZEN_TRACE_CPU("OrchestratorService::GetWorkerList");
 	CbObjectWriter Cbo;
@@ -71,6 +71,10 @@ OrchestratorService::GetWorkerList()
 				Cbo << "ws_connected" << true;
 			}
 			Cbo << "dt" << Worker.LastSeen.GetElapsedTimeMs();
+			if (Annotate)
+			{
+				Annotate(WorkerId, Cbo);
+			}
 			Cbo.EndObject();
 		}
 	});
@@ -144,6 +148,12 @@ OrchestratorService::AnnounceWorker(const WorkerAnnouncement& Ann)
 	}
 }
 
+void
+OrchestratorService::SetProvisionerStateProvider(IProvisionerStateProvider* Provider)
+{
+	m_Provisioner.store(Provider, std::memory_order_release);
+}
+
 bool
 OrchestratorService::IsWorkerWebSocketEnabled() const
 {
@@ -607,6 +617,14 @@ OrchestratorService::ProbeThreadFunction()
 				continue;
 			}
 
+			// Check if the provisioner knows this worker is draining — if so,
+			// unreachability is expected and should not be logged as a warning.
+			bool IsDraining = false;
+			if (IProvisionerStateProvider* Prov = m_Provisioner.load(std::memory_order_acquire))
+			{
+				IsDraining = Prov->GetAgentStatus(Snap.Id) == AgentProvisioningStatus::Draining;
+			}
+
 			ReachableState NewState = ReachableState::Unreachable;
 
 			try
@@ -621,7 +639,10 @@ OrchestratorService::ProbeThreadFunction()
 			}
 			catch (const std::exception& Ex)
 			{
-				ZEN_WARN("probe failed for worker {} ({}): {}", Snap.Id, Snap.Uri, Ex.what());
+				if (!IsDraining)
+				{
+					ZEN_WARN("probe failed for worker {} ({}): {}", Snap.Id, Snap.Uri, Ex.what());
+				}
 			}
 
 			ReachableState PrevState = ReachableState::Unknown;
@@ -646,6 +667,10 @@ OrchestratorService::ProbeThreadFunction()
 						{
 							ZEN_INFO("worker {} ({}) is now reachable", Snap.Id, Snap.Uri);
 						}
+						else if (IsDraining)
+						{
+							ZEN_INFO("worker {} ({}) shut down (draining)", Snap.Id, Snap.Uri);
+						}
 						else if (PrevState == ReachableState::Reachable)
 						{
 							ZEN_WARN("worker {} ({}) is no longer reachable", Snap.Id, Snap.Uri);