fix(gp2rs): write arrangement XML as UTF-8 so GP import survives non-ASCII metadata (#984)

The GP→arrangement-XML writers persisted their output with
`Path.write_text(xml_str)` and no explicit encoding. On Windows that uses
the cp1252 default, so a non-ASCII metadata character — e.g. the © in an
album name like "Chrysalis©1982" — was written as the lone byte 0xA9.
The XML is read back as UTF-8 (expat's default), where 0xA9 is an invalid
start byte, so `parse_arrangement` died with:

    xml.etree.ElementTree.ParseError: not well-formed (invalid token): line 10, column 22

and the whole Guitar Pro import failed (HTTP 500). All three arrangement
XML writes (gp2rs.py, gp2rs_gpx.py ×2) now pin encoding="utf-8".

CI runs on Linux (UTF-8 default) so the bug was invisible there; the new
test pins the locale-independent contract at the source level plus a
round-trip of a © album name.


Claude-Session: https://claude.ai/code/session_01EBQCHCNA81E9tHmSDHSe2Q

Signed-off-by: ChrisBeWithYou <christian.a.cowan@gmail.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

lib/gp2rs.py

@@ -2080,7 +2080,7 @@ def convert_file(
         safe_name = track.name.strip().replace(" ", "_").replace("/", "_")
         filename = f"{safe_name}_{arr_name or 'arr'}.xml"
         filepath = out / filename
-        filepath.write_text(xml_str)
+        filepath.write_text(xml_str, encoding="utf-8")
         output_files.append(str(filepath))
 
     return output_files

lib/gp2rs_gpx.py

@@ -1680,7 +1680,7 @@ def convert_file(
             filepath = safe_join(out, filename)
             if filepath is None:
                 raise ValueError(f"unsafe output filename from track name: {track['name']!r}")
-            filepath.write_text(xml_str)
+            filepath.write_text(xml_str, encoding="utf-8")
             output_files.append(str(filepath))
             continue
 
@@ -2108,7 +2108,7 @@ def convert_file(
         filepath = safe_join(out, filename)
         if filepath is None:
             raise ValueError(f"unsafe output filename from track name: {track['name']!r}")
-        filepath.write_text(xml_str)
+        filepath.write_text(xml_str, encoding="utf-8")
         output_files.append(str(filepath))
 
         # Keys/piano tracks additionally get a standard-notation sidecar