Harmonist v1.2.3

Patch release: the final native-Windows determinism fix. With v1.2.1 (stdout
glyphs) and v1.2.2 (catalog file reads), this makes the native-Windows CI job
pass end to end for the first time — both the Linux and Windows jobs are green.

Fixed

• Deterministic index.json across operating systems. build_index.py
  wrote each agent's path field with str(Path(...)), producing backslashes
  on Windows, so build_index.py --check flagged the committed (forward-slash)
  index as stale on the Windows runner. It now uses .as_posix(), matching
  build_manifest.py. No change to index.json on POSIX.