AI: Instant Meshes quad retopology (qtmesh retopo)

[fernandotonon]

Parent epic: #397

Goal

Wrap Instant Meshes (Wenzel Jakob, BSD) for one-click quad retopology of high-poly imports. Outputs clean quad-dominant meshes suitable for animation and further editing.

Scope

• Vendor Instant Meshes core (it's a C++ library, not just a GUI; library extraction may need ~1 day of plumbing).
• CLI: qtmesh retopo model.fbx --target-faces 5000 -o retopo.fbx.
• CLI: qtmesh retopo model.fbx --target-edge-length 0.05 -o retopo.fbx.
• GUI: Edit Mode toolbar button "Retopologize" with target face count slider.
• MCP: retopologize tool with target_faces param.
• Run as a worker-thread job through AIAssistManager (Epic: AI — Local-AI-assisted 3D workflows #397) — can take seconds to minutes on large meshes.
• Progress reporting via existing progressChanged signal pattern.

Acceptance Criteria

• qtmesh retopo high.fbx --target-faces 3000 -o low.fbx produces a quad-dominant mesh near the target count.
• Original UVs are not preserved (documented limitation — pair with qtmesh uv --unwrap afterwards).
• GUI shows progress and is cancellable.
• Unit test: input N triangles → output close to target face count, manifold.
• Sentry breadcrumb ai.assist.retopo recorded.

Effort

~5–7 days, mostly extracting Instant Meshes' core from its GUI app.