[lightningpixel]

Closes #297

Summary

Adds high-level transforms on top of the raw 4×4 /transform endpoint, so "put it on the ground, center it and lay it flat" works in a single chat message — one tool call, one output file.

• recenter — moves the centroid to the origin.
• drop_to_ground — moves the lowest point to y = 0.
• auto_orient — lays the mesh flat: the smallest rotation that turns its thinnest PCA axis vertical. No spin about the vertical axis, never flipped upside down, and meshes without a clear flat side (cube, sphere) are left alone.
• normalize_scale — resizes to size in m / cm / mm (glTF metres), along the longest side or a chosen axis. Scales around the origin, so a centred, grounded mesh stays so.

Changes

• api/services/mesh_ops/transforms.py — each step builds its matrix from the mesh as the previous steps left it. apply_transforms loads once, runs the steps in a fixed order (orient → scale → recenter → ground, whatever order they were requested in) and writes one file. The /transform gizmo endpoint now uses the same transform_file helper, unchanged in behaviour.
• The 4 ops are registered under the transform category with flat params_schema, ready for the edit panel (T-C1).
• POST /optimize/arrange — composes several steps into one file, validating each step's params through the registry (unknown step or bad param → 400).
• Agent: arrange_mesh tool, with a prompt rule to pass every step in one call. Its result includes the new mesh path, so the model can chain another tool on it. MCP server: modly_arrange_mesh.
• Refactor: world-space geometry helpers move to mesh_ops/geometry.py (shared with inspect_mesh); the registry exposes resolve_params.

Testing

• npm run test:py — 257 tests pass (4 skipped). 17 new tests: canonical order on a tilted, offset plate; centroid to origin; vertical-only grounding; cube left alone; bowl not flipped; 20 cm / 5 mm; scaling keeps the mesh grounded; multi-node scenes in world space; textures preserved; input never modified; invalid steps rejected. Router (/arrange in a moved workspace) and agent tests added.
• On Khronos sample models (Duck, Avocado, DamagedHelmet, BoxTextured): textures and UVs preserved, longest side exactly 20 cm, lowest point at 0.
• ~1.6 s for all four steps on 1.3M faces.

Notes

• The viewer already centres and grounds every mesh on display, so recenter and drop_to_ground change the exported file but nothing on screen.
• auto_orient lays a standing figure down — that is what "lay flat" means.
• Exports keep glTF metres, so a 20 cm mesh exported to STL reads as 0.2 mm in a slicer — that's T-D2 / T-H1.