SceneOps: Slice D — Split submesh (by material / by selection / by connected component)

[fernandotonon]

Parent epic: #623

Goal

Split one submesh into multiple — three modes covering the realistic use-cases:

1. By material — a submesh with per-face material tags becomes one submesh per material. (Edge case but useful for some FBX imports.)
2. By selection — the user selects faces in Edit Mode; "Split selection to new submesh" extracts them. The primary user-requested flow ("split the body parts").
3. By connected component — flood-fill mesh adjacency via HalfEdgeMesh; produces one submesh per topologically-connected island. Useful when the imported geometry has multiple discrete parts baked into one submesh.

Scope

• New helpers:
  • SubMeshOps::splitSubMeshByMaterial(mesh, subIndex) -> std::vector<int> newIndices.
  • SubMeshOps::splitSubMeshBySelection(mesh, subIndex, faceIndices, newName?) -> int newSubIndex.
  • SubMeshOps::splitSubMeshByConnectedComponent(mesh, subIndex) -> std::vector<int> newIndices.
• By material:
  • Requires submeshes that have per-face material tags (rare in practice; common in some Assimp re-imports). No-op if all faces share one material.
• By selection (the primary flow):
  • Inputs: a list of face indices (uint32_t triangle indices in the source submesh).
  • Output: a new submesh containing only those triangles, with vertex deduplication on the boundary (interior-only vertices stay; boundary vertices are duplicated so the original submesh remains a valid closed mesh).
  • Inherits the source submesh's material.
  • Plumbs through EditModeController's face selection → qml/SceneTreeNode.qml context-menu command "Split selection to new submesh".
• By connected component:
  • Build a HalfEdgeMesh over the source submesh.
  • Flood-fill from each face to find connected islands.
  • Produce one submesh per island.
  • Discard the source submesh.
• Naming:
  • By material: appended with material name (Body.Skin, Body.Cloth).
  • By selection: user-prompted (default Body.001).
  • By connected component: numeric suffix (Body.001, Body.002, …).
• Bone assignments: copied to whichever new submesh contains each affected vertex.
• Undo: SplitSubMeshCommand captures pre-split state.

Acceptance Criteria

• By-material split correctly separates multi-material submeshes; no-op on single-material.
• By-selection split produces a new submesh containing exactly the selected faces; the source submesh remains valid.
• By-connected-component split produces N submeshes that, when joined, are byte-identical to the original (round-trip).
• Boundary vertex duplication is correct (no shared vertices across the split).
• Bone assignments survive on a skinned fixture.
• Undo restores the original submesh byte-exactly.
• glTF + FBX round-trip preserves the new submeshes.
• Sentry breadcrumbs mesh.submesh.split with {mode, srcIndex, newIndices}.
• Headless-CI tests for each of the three modes.

Effort

~10 days. By-selection is the highest-value mode; ship it first; by-material and by-component can land in follow-up PRs under this slice.