SceneOps: Slice B — Move submesh between meshes

[fernandotonon]

Parent epic: #623

Goal

Move a submesh from mesh A to mesh B with material, vertex format, and bone-binding preservation. Drag-and-drop in the scene tree; right-click → "Move to mesh…" alternative.

Scope

• New static helper SubMeshOps::moveSubMesh(srcMesh, srcSubIdx, targetMesh, *errorMsg) -> bool.
• Vertex format reconciliation (the hard part):
  • If source and target share the exact vertex declaration: copy buffers directly.
  • If declarations differ: re-encode the source submesh into the target's declaration. Common cases:
    • Source has tangents, target doesn't → drop tangents.
    • Target has UV1, source doesn't → fill UV1 with zeros.
    • Vertex-colour mismatch → fill with white.
  • If source is skinned and target is static → reject with a clear error message; offer "convert to static" as a follow-up CLI.
  • If both are skinned but the skeletons differ → reject. (Skeleton retargeting belongs in Anim: Slice F — Animation retargeting between skeletons #523.)
  • If skeletons match by name → reuse bone handles directly.
• Material: copies the source material name to the target's submesh. If the material isn't in the target's resource group, register it via the existing material resource path.
• Mesh::SubMeshNameMap updated on both meshes.
• Mesh reload: rebuild affected entities so the scene reflects the new submesh structure (Entity::_setMesh if available; otherwise destroy + recreate the entity preserving its scene node).
• UI:
  • Drag a SubEntity row in qml/SceneTreeNode.qml, drop onto another Entity row → triggers move.
  • Visual feedback during drag: highlight valid drop targets in green, invalid in red. Tooltip on hover explains why a target is invalid.
  • Context menu "Move to mesh…" opens a small picker dialog listing every valid target entity.
• Undo: MoveSubMeshCommand that captures the full source-mesh state pre-move (including the vertex data) so undo is byte-exact.
• Multi-select: if multiple submeshes are dragged from the same mesh, move them all in one command.

Acceptance Criteria

• Drag-and-drop moves a submesh between two compatible meshes.
• Reconciliation handles tangent / UV1 / vertex-colour mismatches without crashing.
• Skinned-source / static-target combination is rejected with a clear message.
• Skinned move with matching skeleton preserves deformation correctly.
• Material follows the submesh.
• Both meshes' SubMeshNameMap are correct after the move.
• Undo restores both meshes byte-exactly.
• Multi-submesh drag moves all in one undo step.
• glTF + FBX round-trip preserves the new submesh layout.
• Sentry breadcrumb mesh.submesh.move with {srcMesh, dstMesh, subIndex}.
• Headless-CI test on a 2-mesh fixture covering: equal-decl move, declaration mismatch, skinned move, reject cases.

Effort

~9 days. Vertex declaration reconciliation is the schedule risk.