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]>

vminitd/Sources/Cgroup/Cgroup2Manager.swift

@@ -288,6 +288,12 @@ public struct Cgroup2Manager: Sendable {
         try Self.writeValue(path: self.path, value: String(max), fileName: "pids.max")
     }
 
+    /// [Nucleic vendored patch] Hard memory ceiling (`memory.max`) — a host-configured per-exec cap so
+    /// one session can't consume the whole container's memory before its own (oom.group-scoped) OOM.
+    package func setMemoryMax(bytes: UInt64) throws {
+        try Self.writeValue(path: self.path, value: String(bytes), fileName: "memory.max")
+    }
+
     /// [Nucleic vendored patch] Remove this cgroup directory (rmdir). The cgroup must already be empty
     /// of processes and child cgroups. Best-effort partial-setup cleanup for the per-exec layout.
     package func remove() throws {