Shape List hierarchy phase 2: reorder, duplicate group, selection polish

[trailcode]

Summary

Follow-up work after the organizational Shape List outliner (phase 1). Phase 1 already covers groups, current group, DnD reparent, cascade delete/ungroup, visibility, boolean result parenting, and STEP XCAF assembly import as groups. Phase 2 hardens outliner workflows that still feel incomplete without changing leaf BREP or adding parent transforms.

Context (phase 1 done)

• Hierarchical Shape List with organizational groups (parentId / order / isGroup)
• Current group; new primitives/extrudes/revolves land under it
• Group / New group / Ungroup, DnD reparent, cascade delete, Ctrl multi-select
• STEP import preserves XCAF product/assembly groups (unless Union shapes)

Related: #59 (broader compound/assembly modeling; this issue is the next Shape List slice).

Scope

In scope

• Reorder without reparent — drag between siblings (or explicit move up/down) to change sibling_order under the same parent without changing parent_id.
• Duplicate group — deep-copy a group subtree (structure + leaf solids), unique names, undoable.
• Selection / hover polish — e.g. Select children only, clearer empty-group vs solid selection feedback, optional middle-click or context-menu variants for subtree selection.
• Optional stretch: scripting helpers for parent_id / children / group() / ungroup() if low cost.

Out of scope (phase 3)

• Parent transform inheritance (moving a group moves children)
• Boolean feature-history trees (fuse keeps inputs as children)
• Part-local origins / planes
• Cross-sketch parenting

Acceptance criteria

• User can reorder siblings under the same parent without reparenting to another group
• Duplicate group creates an independent deep copy (structure + solids) with undo
• At least one polished subtree-selection path beyond click-group selects all descendant solids
• Docs (docs/usage.md, src/doc/gui.md / shape.md) and CHANGELOG updated
• Unit tests for reorder and duplicate-group (plus any selection helpers that are non-UI)

Related

• Compound / assembly modeling in EzyCad #59 Compound / assembly modeling in EzyCad
• Shape List hierarchy phase 3: parent transforms and advanced assembly model #214 Shape List hierarchy phase 3: parent transforms and advanced assembly model