feat: add export history auto-fill, fix filename prefixes, persist records for all doc types

- Add database-first lookup for equipment/drawing number auto-fill when
  reopening previously exported files
- Remove prefix prepending for named parts (only use prefix for PT## BOM items)
- Create ExportRecord/BomItem/CutTemplate chains for Part and Assembly
  exports, not just Drawings
- Add auto-incrementing item numbers across drawing numbers
- Add content hashing (SHA256) for DXF and PDF versioning with
  stash/archive pattern
- Add EF Core initial migration for ExportDxfDb

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

ExportDXF/Utilities/ContentHasher.cs

@@ -0,0 +1,118 @@
+using System;
+using System.IO;
+using System.Security.Cryptography;
+using System.Text;
+
+namespace ExportDXF.Utilities
+{
+    public static class ContentHasher
+    {
+        /// <summary>
+        /// Computes a SHA256 hash of DXF file content, skipping the HEADER section
+        /// which contains timestamps that change on every save.
+        /// </summary>
+        public static string ComputeDxfContentHash(string filePath)
+        {
+            var text = File.ReadAllText(filePath);
+            var contentStart = FindEndOfHeader(text);
+            var content = contentStart >= 0 ? text.Substring(contentStart) : text;
+
+            using (var sha = SHA256.Create())
+            {
+                var bytes = sha.ComputeHash(Encoding.UTF8.GetBytes(content));
+                return BitConverter.ToString(bytes).Replace("-", "").ToLowerInvariant();
+            }
+        }
+
+        /// <summary>
+        /// Computes a SHA256 hash of the entire file contents (for PDFs and other binary files).
+        /// </summary>
+        public static string ComputeFileHash(string filePath)
+        {
+            using (var sha = SHA256.Create())
+            using (var stream = File.OpenRead(filePath))
+            {
+                var bytes = sha.ComputeHash(stream);
+                return BitConverter.ToString(bytes).Replace("-", "").ToLowerInvariant();
+            }
+        }
+
+        /// <summary>
+        /// Finds the position immediately after the HEADER section's ENDSEC marker.
+        /// DXF HEADER format:
+        ///   0\nSECTION\n2\nHEADER\n...variables...\n0\nENDSEC\n
+        /// Returns -1 if no HEADER section is found.
+        /// </summary>
+        private static int FindEndOfHeader(string text)
+        {
+            // Find the HEADER section start
+            var headerIndex = FindGroupCode(text, 0, "2", "HEADER");
+            if (headerIndex < 0)
+                return -1;
+
+            // Advance past the HEADER value line so pair scanning stays aligned
+            var headerLineEnd = text.IndexOf('\n', headerIndex);
+            if (headerLineEnd < 0)
+                return -1;
+
+            // Find the ENDSEC that closes the HEADER section
+            var pos = headerLineEnd + 1;
+            while (pos < text.Length)
+            {
+                var endsecIndex = FindGroupCode(text, pos, "0", "ENDSEC");
+                if (endsecIndex < 0)
+                    return -1;
+
+                // Move past the ENDSEC line
+                var lineEnd = text.IndexOf('\n', endsecIndex);
+                return lineEnd >= 0 ? lineEnd + 1 : text.Length;
+            }
+
+            return -1;
+        }
+
+        /// <summary>
+        /// Finds a DXF group code pair (code line followed by value line) starting from the given position.
+        /// Returns the position of the value line, or -1 if not found.
+        /// </summary>
+        private static int FindGroupCode(string text, int startIndex, string groupCode, string value)
+        {
+            var pos = startIndex;
+            while (pos < text.Length)
+            {
+                // Skip whitespace/newlines to find the group code
+                while (pos < text.Length && (text[pos] == '\r' || text[pos] == '\n' || text[pos] == ' '))
+                    pos++;
+
+                if (pos >= text.Length)
+                    break;
+
+                // Read the group code line
+                var codeLineEnd = text.IndexOf('\n', pos);
+                if (codeLineEnd < 0)
+                    break;
+
+                var codeLine = text.Substring(pos, codeLineEnd - pos).Trim();
+
+                // Move to the value line
+                var valueStart = codeLineEnd + 1;
+                if (valueStart >= text.Length)
+                    break;
+
+                var valueLineEnd = text.IndexOf('\n', valueStart);
+                if (valueLineEnd < 0)
+                    valueLineEnd = text.Length;
+
+                var valueLine = text.Substring(valueStart, valueLineEnd - valueStart).Trim();
+
+                if (codeLine == groupCode && string.Equals(valueLine, value, StringComparison.OrdinalIgnoreCase))
+                    return valueStart;
+
+                // Move to the next pair
+                pos = valueLineEnd + 1;
+            }
+
+            return -1;
+        }
+    }
+}