feat: port session 01 — Foundation Types (const, errors, proto, ring, rate_counter, sdm)

Ports server/const.go, errors.go, proto.go, ring.go, rate_counter.go, sdm.go.
- ServerConstants: all protocol constants and version info from const.go
- ServerErrors: ~60 sentinel exceptions plus errCtx/configErr/processConfigErr types
- ProtoWire: protobuf varint encode/decode helpers (proto.go)
- RateCounter: sliding-window rate limiter (rate_counter.go)
- ClosedRingBuffer: fixed-size ring buffer for /connz (ring.go)
- StreamDeletionMeta: SDM tracking for JetStream cluster consensus (sdm.go)
- 5 unit tests passing (errors, ring buffer, rate counter)
- errors_gen.go (code generator tool) and nkey.go Server methods marked n_a

dotnet/src/ZB.MOM.NatsNet.Server/Internal/DataStructures/StreamDeletionMeta.cs

@@ -0,0 +1,121 @@
+// Copyright 2025 The NATS Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// Adapted from server/sdm.go in the NATS server Go source.
+
+namespace ZB.MOM.NatsNet.Server.Internal.DataStructures;
+
+/// <summary>
+/// Per-sequence data tracked by <see cref="StreamDeletionMeta"/>.
+/// Mirrors <c>SDMBySeq</c> in server/sdm.go.
+/// </summary>
+public readonly struct SdmBySeq
+{
+    /// <summary>Whether this sequence was the last message for its subject.</summary>
+    public bool Last { get; init; }
+
+    /// <summary>Timestamp (nanoseconds UTC) when the removal/SDM was last proposed.</summary>
+    public long Ts { get; init; }
+}
+
+/// <summary>
+/// Tracks pending subject delete markers (SDMs) and message removals for a stream.
+/// Used by JetStream cluster consensus to avoid redundant proposals.
+/// Mirrors <c>SDMMeta</c> in server/sdm.go.
+/// </summary>
+public sealed class StreamDeletionMeta
+{
+    // Per-subject pending-count totals.
+    private readonly Dictionary<string, ulong> _totals = new(1);
+
+    // Per-sequence data keyed by sequence number.
+    private readonly Dictionary<ulong, SdmBySeq> _pending = new(1);
+
+    // -------------------------------------------------------------------------
+    // Header constants (forward-declared; populated in session 19 — JetStream).
+    // isSubjectDeleteMarker checks these header keys.
+    // -------------------------------------------------------------------------
+
+    // Mirrors JSMarkerReason header key (defined in jetstream.go).
+    internal const string HeaderJsMarkerReason = "Nats-Marker-Reason";
+
+    // Mirrors KVOperation header key (defined in jetstream.go).
+    internal const string HeaderKvOperation = "KV-Operation";
+
+    // Mirrors KVOperationValuePurge (defined in jetstream.go).
+    internal const string KvOperationValuePurge = "PURGE";
+
+    /// <summary>
+    /// Returns true when the given header block contains a subject delete marker
+    /// (either a JetStream marker or a KV purge operation).
+    /// Mirrors <c>isSubjectDeleteMarker</c> in server/sdm.go.
+    /// </summary>
+    public static bool IsSubjectDeleteMarker(ReadOnlySpan<byte> hdr)
+    {
+        // Simplified header scan: checks whether JSMarkerReason key is present
+        // or whether KV-Operation equals "PURGE".
+        // Full implementation depends on SliceHeader from session 08 (client.go).
+        // Until then this provides the correct contract.
+        var text = System.Text.Encoding.UTF8.GetString(hdr);
+        if (text.Contains(HeaderJsMarkerReason))
+            return true;
+        if (text.Contains($"{HeaderKvOperation}: {KvOperationValuePurge}"))
+            return true;
+        return false;
+    }
+
+    /// <summary>
+    /// Clears all tracked data.
+    /// Mirrors <c>SDMMeta.empty</c>.
+    /// </summary>
+    public void Empty()
+    {
+        _totals.Clear();
+        _pending.Clear();
+    }
+
+    /// <summary>
+    /// Tracks <paramref name="seq"/> as pending and returns whether it was
+    /// the last message for its subject.  If the sequence is already tracked
+    /// the existing <c>Last</c> value is returned without modification.
+    /// Mirrors <c>SDMMeta.trackPending</c>.
+    /// </summary>
+    public bool TrackPending(ulong seq, string subj, bool last)
+    {
+        if (_pending.TryGetValue(seq, out var p))
+            return p.Last;
+
+        _pending[seq] = new SdmBySeq { Last = last, Ts = DateTimeOffset.UtcNow.ToUnixTimeMilliseconds() * 1_000_000L };
+        _totals[subj] = _totals.TryGetValue(subj, out var cnt) ? cnt + 1 : 1;
+        return last;
+    }
+
+    /// <summary>
+    /// Removes <paramref name="seq"/> and decrements the pending count for
+    /// <paramref name="subj"/>, deleting the subject entry when it reaches zero.
+    /// Mirrors <c>SDMMeta.removeSeqAndSubject</c>.
+    /// </summary>
+    public void RemoveSeqAndSubject(ulong seq, string subj)
+    {
+        if (!_pending.Remove(seq))
+            return;
+
+        if (_totals.TryGetValue(subj, out var msgs))
+        {
+            if (msgs <= 1)
+                _totals.Remove(subj);
+            else
+                _totals[subj] = msgs - 1;
+        }
+    }
+}