Surface lastExitCode on ContainerSnapshot

Adds an optional lastExitCode: Int32? field to ContainerSnapshot,
populated in ContainersService.handleContainerExit when the container
transitions to .stopped. The exit code is taken from the existing
ExitMonitor callback's ExitStatus value.

This unblocks Container-Compose's depends_on:
  condition: service_completed_successfully

which currently has to fall back to a stopped-only check because the
SDK doesn't surface the underlying process exit code.

Wire format note: ContainerSnapshot is marshaled as Codable JSON over
XPC. Adding an optional field is forward-compatible; older clients
will silently ignore the new key, and newer clients reading from older
servers will decode lastExitCode as nil.

Scope: in-memory only. The exit code lives in the in-memory ContainerState
snapshot for the duration of apiserver uptime. A daemon restart resets
all snapshots to .stopped without exit codes (existing behavior). A
follow-up patch can add bundle persistence (exit_status.json) if needed
to survive restarts.

Files:
- Sources/ContainerResource/Container/ContainerSnapshot.swift: new
  field + init parameter (default nil for backward compat at all
  existing construction sites).
- Sources/Services/ContainerAPIService/Server/Containers/ContainersService.swift:
  one assignment in handleContainerExit's terminal state block.

Author: chrisgeo

Sources/ContainerResource/Container/ContainerSnapshot.swift

@@ -39,16 +39,22 @@ public struct ContainerSnapshot: Codable, Sendable {
     public var networks: [Attachment]
     /// When the container was started.
     public var startedDate: Date?
+    /// Last observed exit code from the container's main process.
+    /// Populated when the container transitions to ``RuntimeStatus/stopped``.
+    /// `nil` if the container has never exited or its exit was not captured.
+    public var lastExitCode: Int32?
 
     public init(
         configuration: ContainerConfiguration,
         status: RuntimeStatus,
         networks: [Attachment],
-        startedDate: Date? = nil
+        startedDate: Date? = nil,
+        lastExitCode: Int32? = nil
     ) {
         self.configuration = configuration
         self.status = status
         self.networks = networks
         self.startedDate = startedDate
+        self.lastExitCode = lastExitCode
     }
 }

Sources/Services/ContainerAPIService/Server/Containers/ContainersService.swift

@@ -1011,6 +1011,7 @@ public actor ContainersService {
         }
 
         state.snapshot.status = .stopped
+        state.snapshot.lastExitCode = code?.exitCode
         state.snapshot.networks = []
         state.client = nil
         state.allocatedAttachments = []