Add DigitalData.Auth.Claims project and EnvelopeClaimNames

Added a new .NET project `DigitalData.Auth.Claims` targeting `net8.0`
with metadata for NuGet packaging. The project is configured to
not generate a NuGet package on build.

Introduced the `EnvelopeClaimNames` static class to define strongly-typed
constants for custom JWT claim names specific to envelope receiver tokens.
These include `EnvelopeId`, `EnvelopeUuid`, `ReceiverId`, and
`ReceiverSignature`. Added XML documentation for all constants.

DigitalData.Auth.Claims/EnvelopeClaimNames.cs

@@ -0,0 +1,34 @@
+namespace DigitalData.Auth.Claims
+{
+    /// <summary>
+    /// Defines the custom JWT claim names used for envelope receiver tokens.
+    /// These claims are domain-specific and have no equivalent in <see cref="Microsoft.IdentityModel.JsonWebTokens.JwtRegisteredClaimNames"/>.
+    /// </summary>
+    public static class EnvelopeClaimNames
+    {
+        /// <summary>
+        /// The database primary key of the envelope.
+        /// Claim name: <c>envelope_id</c>
+        /// </summary>
+        public const string EnvelopeId = "envelope_id";
+
+        /// <summary>
+        /// The unique identifier (UUID) of the envelope.
+        /// Claim name: <c>envelope_uuid</c>
+        /// </summary>
+        public const string EnvelopeUuid = "envelope_uuid";
+
+        /// <summary>
+        /// The database primary key of the envelope receiver.
+        /// Claim name: <c>receiver_id</c>
+        /// </summary>
+        public const string ReceiverId = "receiver_id";
+
+        /// <summary>
+        /// The cryptographic signature of the envelope receiver.
+        /// Together with <see cref="EnvelopeUuid"/>, it forms the unique envelope key via <c>ToEnvelopeKey()</c>.
+        /// Claim name: <c>receiver_sig</c>
+        /// </summary>
+        public const string ReceiverSignature = "receiver_sig";
+    }
+}