The editor's keys LH/RH hand arc needs a per-note hand assignment
('lh'/'rh', from a MusicXML grand staff import today, hand-editable
later) to survive a sloppak save → reload: the editor already emits it
on the wire, but note_from_wire dropped unknown keys, so the field died
on every reopen.
- Note gains `hand: str | None = None` (None = unassigned; the
heuristic hand split keeps owning unassigned notes). Distinct from
`right_hand` (the bass plucking finger) — hence the spelled-out
`hand` wire key, since `rh` is taken.
- note_to_wire emits it default-omitted and validates on emit; older
readers ignore it (feedpak: unknown note keys are permitted).
- note_from_wire decodes it as a strict enum — anything but 'lh'/'rh'
(junk, wrong case, bools) falls back to unassigned rather than
poisoning downstream hand-split / hands-separate practice logic.
Groundwork consumers land separately: notation_lift.split_hands
respecting per-note overrides, and the editor's hand surface.
Editor counterpart: feedBack-plugin-editor #299.
Tests: three new wire round-trip tests in tests/test_song.py (literal
key, default-omitted, junk rejection both directions), matching the
teaching-marks test style.
Claude-Session: https://claude.ai/code/session_01EBQCHCNA81E9tHmSDHSe2Q
Signed-off-by: ChrisBeWithYou <chris@rifflarr.local>
Co-authored-by: ChrisBeWithYou <chris@rifflarr.local>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>
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