feat(posts): CI Fiber one program per sheet, configurable pallet change

Multi-sheet nests previously went into one program with a single header
size and one M50 at the end, so sheet 2 would cut into sheet 1's
skeleton. New Sheets settings:

- One program per sheet (default on): JOB.cnc -> JOB-1.cnc, JOB-2.cnc,
  each a full program with its own size and pallet change (CL-series
  batch rule, EM-423 7.4). Single-sheet nests keep the chosen name.
- Off: one program, with /L "L0" + pallet change between sheets;
  mixed sheet sizes are rejected.
- Pallet change code (default M50, unconfirmed for multi-sheet CI Fiber
  runs; documented as a release blocker).

All sheets are validated and rendered before any file is written.
IMultiFilePostProcessor lets the desktop app confirm overwrites of every
target file and list what was saved, and the console print each file.

Posts/OpenNest.Posts.CincinnatiCIFiber/CIFiberPostProcessor.cs

@@ -16,7 +16,9 @@ namespace OpenNest.Posts.CincinnatiCIFiber
     /// the configured table envelope. Emits the TF5200 skippable-macro program
     /// structure of the machine sample (see <see cref="CIFiberProgramWriter"/>).
     /// </summary>
-    public sealed class CIFiberPostProcessor : IConfigurablePostProcessor
+    public sealed class CIFiberPostProcessor
+        : IConfigurablePostProcessor,
+            IMultiFilePostProcessor
     {
         private static readonly JsonSerializerOptions JsonOptions = new()
         {
@@ -70,6 +72,11 @@ namespace OpenNest.Posts.CincinnatiCIFiber
             return Path.Combine(dir, name + ".json");
         }
 
+        /// <summary>
+        /// Writes every sheet into one program. A stream holds a single program,
+        /// so this throws when "One program per sheet" is on and the nest has more
+        /// than one sheet; use <see cref="Post(Nest, string)"/> for that.
+        /// </summary>
         public void Post(Nest nest, Stream outputStream)
         {
             if (nest == null)
@@ -77,26 +84,86 @@ namespace OpenNest.Posts.CincinnatiCIFiber
             if (outputStream == null)
                 throw new ArgumentNullException(nameof(outputStream));
 
-            // CRLF file, ASCII (UTF-8 without BOM), matching the machine sample.
-            var encoding = new UTF8Encoding(false);
-            using var writer = new StreamWriter(
-                outputStream,
-                encoding,
-                1024,
-                leaveOpen: true
-            );
-            new CIFiberProgramWriter(Config).Write(nest, writer);
-            writer.Flush();
+            if (SplitsSheets(nest))
+                throw new InvalidOperationException(
+                    "\"One program per sheet\" writes several files; post this nest to a file path."
+                );
+
+            var program = Render(w => new CIFiberProgramWriter(Config).Write(nest, w));
+            outputStream.Write(program, 0, program.Length);
         }
 
+        /// <summary>
+        /// Writes the program(s) for the nest. With "One program per sheet" and more
+        /// than one sheet, <c>JOB.cnc</c> becomes <c>JOB-1.cnc</c>, <c>JOB-2.cnc</c>, ...;
+        /// otherwise the single program goes to <paramref name="outputFile"/>. All
+        /// programs are generated before any file is written, so a validation
+        /// failure leaves no partial output.
+        /// </summary>
         public void Post(Nest nest, string outputFile)
         {
-            using var fs = new FileStream(
-                outputFile,
-                FileMode.Create,
-                FileAccess.Write
-            );
-            Post(nest, fs);
+            if (nest == null)
+                throw new ArgumentNullException(nameof(nest));
+            if (string.IsNullOrEmpty(outputFile))
+                throw new ArgumentNullException(nameof(outputFile));
+
+            var files = GetOutputFiles(nest, outputFile);
+            var programs = new List<byte[]>();
+
+            if (files.Count == 1)
+            {
+                programs.Add(Render(w => new CIFiberProgramWriter(Config).Write(nest, w)));
+            }
+            else
+            {
+                var sheets = CIFiberProgramWriter.PostedSheets(nest);
+                var writer = new CIFiberProgramWriter(Config);
+                writer.Validate(sheets, singleProgram: false);
+                for (var i = 0; i < sheets.Count; i++)
+                {
+                    var sheetNumber = i + 1;
+                    var sheet = sheets[i];
+                    programs.Add(Render(w => writer.WriteSheet(nest, sheet, sheetNumber, w)));
+                }
+            }
+
+            for (var i = 0; i < files.Count; i++)
+                File.WriteAllBytes(files[i], programs[i]);
+        }
+
+        public IReadOnlyList<string> GetOutputFiles(Nest nest, string outputFile)
+        {
+            if (nest == null)
+                throw new ArgumentNullException(nameof(nest));
+            if (string.IsNullOrEmpty(outputFile))
+                throw new ArgumentNullException(nameof(outputFile));
+
+            if (!SplitsSheets(nest))
+                return new[] { outputFile };
+
+            var dir = Path.GetDirectoryName(outputFile) ?? "";
+            var name = Path.GetFileNameWithoutExtension(outputFile);
+            var ext = Path.GetExtension(outputFile);
+            var count = CIFiberProgramWriter.PostedSheets(nest).Count;
+
+            return Enumerable
+                .Range(1, count)
+                .Select(n => Path.Combine(dir, $"{name}-{n}{ext}"))
+                .ToList();
+        }
+
+        private bool SplitsSheets(Nest nest) =>
+            Config.OneProgramPerSheet && CIFiberProgramWriter.PostedSheets(nest).Count > 1;
+
+        // CRLF file, ASCII (UTF-8 without BOM), matching the machine sample.
+        private static byte[] Render(Action<TextWriter> write)
+        {
+            using var ms = new MemoryStream();
+            using (var writer = new StreamWriter(ms, new UTF8Encoding(false), 1024, leaveOpen: true))
+            {
+                write(writer);
+            }
+            return ms.ToArray();
         }
     }
 }