The bottom handle bar is now dynamic. Fixed CP + FO handles, then one
handle per kind of sub-object the focused node has. "Kind" is the SLOT
part of the AST's SLOT.SUBTYPE namespace: sub-objects sharing a SLOT
(e.g. `line.stem_note` + `line.motif`, or `articles.<subtypeA>` +
`articles.<subtypeB>`) collapse into one pane; different SLOTs split.
Score-level kinds emit, in this order: TU (tuning), ST (stage cone +
voices), IN (instruments — linked ones listed read-only), AR (articles),
BA (bars). info metadata is folded into the FO pane (was previously
listed as a sub-object).
Handles are 2-letter labels for now, intended as alt-text for icons in
a later release.
- `subobject-kinds.js` new: `getKindGroups(node)` returns ordered
`[{ kind, items }]`; `KIND_LABEL` maps kind -> 2-letter label.
- `PaneSubObjects.js` now parameterized by `kind` prop; renders one group.
- `AppShell.js` builds the handle bar from `getKindGroups(focusedNode)`;
drops back to FO when the active sub-pane disappears after a focus change.
- `ObjectShort.js` accepts a `readOnly` flag (used for linked instruments).
