Per-exec cgroups follow-up: host-configured hard memory.max (no protobuf)

Adds an opt-in hard per-session memory ceiling on top of patch #9's scoped-OOM.
The exec already ships the full OCI Spec, so the limit rides
spec.linux.resources.memory.limit — no RPC/protobuf change:

- host framework: LinuxProcessConfiguration.memoryLimitInBytes; LinuxContainer.exec
  stamps it onto the exec spec.
- guest: Server+GRPC.createProcess reads it back and applies it as the exec
  cgroup's memory.max (new Cgroup2Manager.setMemoryMax) via createExec/ManagedProcess.
- Nucleic: ContainerServiceSettings.controlPerSessionMemoryGiB (default 0 = off),
  applied only to the shared control container (ContainerManager.exec); wired
  through ContainerEngine.exec.

So one session can't consume the whole shared container's memory before its own
(oom.group-scoped) OOM. Default off preserves #9's behavior. Compile-verified host
+ musl guest; rides the pending -nucleic2 image, still runtime-pending.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

Sources/Containerization/LinuxContainer.swift

@@ -911,6 +911,11 @@ extension LinuxContainer {
             var config = LinuxProcessConfiguration()
             try configuration(&config)
             spec.process = config.toOCI()
+            // [Nucleic vendored patch] Per-exec memory ceiling → the exec's OCI resources, which the
+            // guest applies as memory.max on this exec's own cgroup (patch #9).
+            if let limit = config.memoryLimitInBytes {
+                spec.linux?.resources?.memory?.limit = Int64(limit)
+            }
 
             let stdio = IOUtil.setup(
                 portAllocator: self.hostVsockPorts,
@@ -948,6 +953,10 @@ extension LinuxContainer {
 
             var spec = self.generateRuntimeSpec()
             spec.process = configuration.toOCI()
+            // [Nucleic vendored patch] Per-exec memory ceiling → the exec's OCI resources (see above).
+            if let limit = configuration.memoryLimitInBytes {
+                spec.linux?.resources?.memory?.limit = Int64(limit)
+            }
 
             let stdio = IOUtil.setup(
                 portAllocator: self.hostVsockPorts,

Sources/Containerization/LinuxProcessConfiguration.swift

@@ -388,6 +388,11 @@ public struct LinuxProcessConfiguration: Sendable {
     public var stdout: Writer?
     /// The stderr for the process.
     public var stderr: Writer?
+    /// [Nucleic vendored patch] A hard per-exec memory ceiling in bytes. When set, `LinuxContainer.exec`
+    /// stamps it onto the exec's `spec.linux.resources.memory.limit`, which the guest applies as
+    /// `memory.max` on this exec's own cgroup (see patch #9) — so one session can't consume the whole
+    /// shared container's memory before *its own* OOM. nil → no per-exec ceiling (inherit the container).
+    public var memoryLimitInBytes: UInt64?
 
     public init() {}