feat(server): implement sealed share lifecycle

Signed-off-by: JustMe <mr.toing@outlook.com>

server/src/WebDrop.Core/Envelope.cs

@@ -0,0 +1,50 @@
+using System.Buffers.Binary;
+
+namespace WebDrop.Core;
+
+/// <summary>帧封装的 26 字节明文头部,见 protocol.md §4。负载由服务端原样保存,从不解析。</summary>
+public readonly record struct Envelope(FrameFlags Flags, Id16 ItemId, uint FrameIndex, int PayloadLength)
+{
+    public const int HeaderSize = 26;
+    public const byte Version = 1;
+
+    public int TotalLength => HeaderSize + PayloadLength;
+
+    /// <summary>头部格式合法、保留位为 0、长度与 payload_len 严格一致。</summary>
+    public static bool TryParse(ReadOnlySpan<byte> data, out Envelope envelope)
+    {
+        envelope = default;
+        if (data.Length < HeaderSize || data[0] != Version) return false;
+
+        byte flags = data[1];
+        if ((flags & 0xF0) != 0) return false;
+
+        uint payloadLen = BinaryPrimitives.ReadUInt32BigEndian(data[22..]);
+        if (payloadLen > int.MaxValue || data.Length != HeaderSize + (int)payloadLen) return false;
+
+        envelope = new Envelope(
+            (FrameFlags)flags,
+            Id16.FromBytes(data.Slice(2, Id16.Size)),
+            BinaryPrimitives.ReadUInt32BigEndian(data[18..]),
+            (int)payloadLen);
+        return true;
+    }
+
+    public bool IsFinal => (Flags & FrameFlags.Final) != 0;
+    public bool IsGated => (Flags & FrameFlags.Gated) != 0;
+    public bool IsSnapshot => (Flags & FrameFlags.Snapshot) != 0;
+    public bool IsEnc => (Flags & FrameFlags.Enc) != 0;
+
+    /// <summary>条目级标志:同一条目的所有帧必须一致。</summary>
+    public FrameFlags ItemFlags => Flags & (FrameFlags.Snapshot | FrameFlags.Gated);
+}
+
+public static class PayloadTiers
+{
+    public const int TagSize = 16;
+    private static readonly int[] Tiers = [1024, 4096, 16384, 65536, 262144, 1048576];
+
+    /// <summary>enc 负载 = 档位明文 + 16 字节 GCM 标签,服务端据此强制 padding。</summary>
+    public static bool IsValidEncPayloadLength(int payloadLength) =>
+        payloadLength > TagSize && Array.IndexOf(Tiers, payloadLength - TagSize) >= 0;
+}