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>

CHANGELOG.md

@@ -229,6 +229,13 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   engine (`app.js`, `highway.js`, `playSong`, `showScreen`, the capability registry).
 
 ### Fixed
+- **Guitar Pro import no longer fails on non-ASCII song metadata (Windows).**
+  The GP→arrangement-XML writers wrote their output with `Path.write_text()`
+  and no explicit encoding, so on Windows (cp1252 default) a metadata
+  character like the © in an album name ("Chrysalis©1982") was written as a
+  lone `0xA9` byte — invalid UTF-8 — and import died with
+  `not well-formed (invalid token): line N, column 22`. All three arrangement
+  XML writes now pin `encoding="utf-8"`.
 - **3D Highway: the lane stops at the hit line** (#991) — the highway lane, its
   dividers, and the fret boundary extension lines ran `BEHIND` seconds *past* the
   hit line toward the player. Nothing is ever drawn in that strip (notes and chord