feat: add container snapshot and SDN subnet cmdlets

Container snapshots (4 cmdlets):
- Get-PveContainerSnapshot, New-PveContainerSnapshot
- Remove-PveContainerSnapshot, Restore-PveContainerSnapshot
- Service methods in ContainerService
- Pester unit tests

SDN subnet management (3 cmdlets):
- Get-PveSdnSubnet, New-PveSdnSubnet, Remove-PveSdnSubnet
- PveSdnSubnet model and NetworkService methods
- Pester unit tests

Also updates manifest (73 cmdlets), README cmdlet reference, and CHANGELOG.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/PSProxmoxVE.Core/Models/Network/PveSdnSubnet.cs

@@ -0,0 +1,75 @@
+using System.Text.Json.Serialization;
+using Newtonsoft.Json;
+
+namespace PSProxmoxVE.Core.Models.Network;
+
+/// <summary>
+/// Represents a Software-Defined Networking (SDN) subnet as returned by
+/// the /cluster/sdn/vnets/{vnet}/subnets endpoint.
+/// Available in Proxmox VE 8.0+.
+/// </summary>
+public class PveSdnSubnet
+{
+    /// <summary>
+    /// The subnet CIDR (e.g. "10.0.0.0/24" or "2001:db8::/64").
+    /// </summary>
+    [JsonPropertyName("subnet")]
+    [JsonProperty("subnet")]
+    public string Subnet { get; set; } = string.Empty;
+
+    /// <summary>
+    /// The VNet this subnet belongs to.
+    /// </summary>
+    [JsonPropertyName("vnet")]
+    [JsonProperty("vnet")]
+    public string? Vnet { get; set; }
+
+    /// <summary>
+    /// The gateway IP address for this subnet.
+    /// </summary>
+    [JsonPropertyName("gateway")]
+    [JsonProperty("gateway")]
+    public string? Gateway { get; set; }
+
+    /// <summary>
+    /// Whether SNAT (source NAT) is enabled for this subnet.
+    /// </summary>
+    [JsonPropertyName("snat")]
+    [JsonProperty("snat")]
+    public int? Snat { get; set; }
+
+    /// <summary>
+    /// The DNS zone name for this subnet.
+    /// </summary>
+    [JsonPropertyName("dnszoneprefix")]
+    [JsonProperty("dnszoneprefix")]
+    public string? DnsZonePrefix { get; set; }
+
+    /// <summary>
+    /// The DHCP range configuration for automatic IP assignment.
+    /// </summary>
+    [JsonPropertyName("dhcp-range")]
+    [JsonProperty("dhcp-range")]
+    public string? DhcpRange { get; set; }
+
+    /// <summary>
+    /// The subnet type identifier used internally by PVE.
+    /// </summary>
+    [JsonPropertyName("type")]
+    [JsonProperty("type")]
+    public string? Type { get; set; }
+
+    /// <summary>
+    /// Optional comment or description.
+    /// </summary>
+    [JsonPropertyName("comments")]
+    [JsonProperty("comments")]
+    public string? Comments { get; set; }
+
+    /// <inheritdoc />
+    public override string ToString()
+    {
+        return $"SDN Subnet: {Subnet} | VNet: {Vnet ?? "N/A"} | "
+             + $"Gateway: {Gateway ?? "N/A"}";
+    }
+}