fix(localdb): fault live sync session on cap-prune; keepalive + newest-wins sync guard (0.3.2, ScadaBridge arch-review P4-1 + P4-3)

ZB.MOM.WW.LocalDb/src/ZB.MOM.WW.LocalDb.Replication/Internal/OplogStore.cs

@@ -24,8 +24,9 @@ internal sealed class OplogStore(ILocalDb db, ReplicationOptions options, Func<D
 
     /// <summary>
     /// Reads up to <paramref name="maxBatch"/> oplog entries with <c>seq &gt; afterSeq</c> in ascending
-    /// seq order. Caller contract: check <c>needs_snapshot</c> before trusting incremental reads —
-    /// after cap-pruning, reads silently resume past a gap.
+    /// seq order. Caller contract: this read resumes SILENTLY past a cap-pruned gap, so a caller
+    /// streaming incrementally must first confirm its position is still below the horizon — see
+    /// <see cref="GetMinSeqAsync"/>, which <c>SyncSession</c>'s pump checks on every iteration.
     /// </summary>
     public Task<IReadOnlyList<OplogEntryRecord>> ReadBatchAboveAsync(long afterSeq, int maxBatch, CancellationToken ct = default) =>
         db.QueryAsync(
@@ -148,6 +149,24 @@ internal sealed class OplogStore(ILocalDb db, ReplicationOptions options, Func<D
         return rows[0];
     }
 
+    /// <summary>
+    /// The oldest oplog seq still present — the delta horizon — or 0 when the oplog is empty.
+    /// </summary>
+    /// <remarks>
+    /// One b-tree seek (<c>seq</c> is the rowid and SQLite optimizes MIN into a seek), which is what
+    /// makes it affordable for the pump to check on every flush interval. Compared against a
+    /// session's sent watermark it answers the only question the pump has: is everything I have not
+    /// yet sent still here? Ack-pruning deletes <c>seq &lt;= last_acked_seq</c> and the peer can only
+    /// ack what it received, so in a healthy session the horizon never climbs past
+    /// <c>sentThruSeq + 1</c>; a higher one means a cap prune ate unsent deltas.
+    /// </remarks>
+    public async Task<long> GetMinSeqAsync(CancellationToken ct = default)
+    {
+        var rows = await db.QueryAsync(
+            "SELECT COALESCE(MIN(seq), 0) FROM __localdb_oplog", static r => r.GetInt64(0), null, ct);
+        return rows[0];
+    }
+
     /// <summary>The highest oplog seq present, or 0 when the oplog is empty.</summary>
     public async Task<long> GetMaxSeqAsync(CancellationToken ct = default)
     {