investigate 4-byte length prefix on Mac for Mac-XM* clipboard payloads

[fuzzzerd]

Summary

SharpFM unconditionally prepends a 4-byte little-endian uint32 length header to the XML payload when writing to the clipboard, and unconditionally strips 4 bytes when reading. This matches FileMaker Pro's wire format on Windows (confirmed by Windows smoke testing, clips round-trip cleanly). macOS behavior is not yet validated and may differ — if FileMaker Pro on Mac does not use the prefix, SharpFM will currently corrupt round-trips on Mac in both directions.

Where the prefix lives

ClipboardService is a pure byte pipe. All prefix logic is in FileMakerClip:

• Write (src/SharpFM.Model/FileMakerClip.cs:100-112):

  byte[] byteList = Encoding.UTF8.GetBytes(XmlData);
  byte[] intBytes = BitConverter.GetBytes(byteList.Length);
  _cachedRawData = intBytes.Concat(byteList).ToArray();

• Read (src/SharpFM.Model/FileMakerClip.cs:62):

  XmlData = ClipBytesToPrettyXml(data.Skip(4));

Applied to every Mac-XM* format on every platform.

Failure modes if macOS doesn't use the prefix

• Copy (SharpFM → FM on Mac): FileMaker reads 4 extra bytes before the XML — may reject the payload, render garbled, or silently truncate.
• Paste (FM on Mac → SharpFM): SharpFM drops the first 4 bytes of real XML — the opening <fmxmlsnippet> is cut, XDocument.Parse fails or silently loses content.

What needs to happen

1. Validate on macOS. Copy a script out of FileMaker Pro on Mac, paste into SharpFM, confirm the XML is intact (no trailing garbage, no truncated opening tag). Reverse: copy from SharpFM, paste into FileMaker Pro on Mac, confirm the script body imports cleanly.
2. If round-trip fails, make the prefix handling platform-conditional:
   • FileMakerClip.cs:62 — data.Skip(OperatingSystem.IsWindows() ? 4 : 0)
   • FileMakerClip.cs:100-112 — prepend length only when OperatingSystem.IsWindows()
   • (Alternative: move the concern into ClipboardService so FileMakerClip stays a pure data model.)
3. Add regression tests using a real Mac-captured payload fixture from step 1 so the platform split can't silently break later.