chore: drop orphaned math/ tree + codename / dead constants in arch/m…

…odels/config.py

A full sweep of `wt/_core/` for (a) dead modules and (b) leftover
internal codenames in config blobs found two more spots to clean:

* `wt/_core/math/` (the four files `__init__.py`, `coord_utils.py`,
  `quaternion.py`, `rigid_body.py`) is orphaned.  Each file only
  imports from another file inside the same `math/` directory, plus a
  `wt._core.math.safe_math` module that doesn't even exist.  No public
  module under `wt/*.py` or `examples/` ever reaches this tree.
  Deleted the whole directory.

* `wt/_core/arch/models/config.py` still had:
    - `FTGS_TIME_SCALE`, `FTGS_DURATION_SCALE`, `FTGS_VELOCITY_SCALE`,
      `FTGS_ANGULAR_VELOCITY_SCALE`  -- `FTGS` is an internal codename
      and these constants are never read.
    - `POINT_TRACK_KEYS`, `POINT_TRACK_CONF_KEYS`,
      `VOXEL_UNET_CONFIG`, `PROPERTY_LIST_AVG`, `PROPERTY_LIST_INVALID`
      -- gaussian-splatting / point-track config blobs from the
      training pipeline that the inference release doesn't touch.
    - duplicate `MOGE_CONFIG` (the non-FA3 variant) that nobody loads.
  Trimmed to just the four constants actually consumed by
  `MultilayerBackbone` / `wt.model`:
  `MOGE_CONFIG_FA3`, `MOGE_PATCH_SIZE`, `DIFFUSION_TIMESTEP_SCALE`,
  `T_MIN_CLAMP`.

The `# from https://github.com/lab4d-org/lab4d/` source attribution
at the top of `wt/_core/splat/utils/embedding.py` and the
`# Copyright 2023 The Google Research Authors.` headers that used to
live on the deleted `math/*.py` files are external upstream
attribution; the embedding.py header stays for license compliance.

Co-authored-by: Cursor <cursoragent@cursor.com>