Add E57 import and pose write-back to step 2 - #515

Load E57 point clouds into multi_view_tls_registration (step 2) and write
registered poses back into the source .e57 files.

- Vendor Apache Xerces-C (v3.3.0) and libE57Format (v3.2.0) as git
  submodules, built statically from source; cmake/FindXercesC.cmake
  redirects libE57Format's find_package(XercesC) at the in-tree target so
  no system package is needed.
- core/e57_utils: load_e57() decodes every Data3D block (cartesian or
  spherical coords, intensity, RGB, timestamps, embedded pose);
  rewrite_e57_poses() copies a file verbatim (points, all point fields,
  line groups, 2D images) while replacing selected Data3D poses, moving
  attached images by the same delta.
- Step 2 GUI: "Open e57" menu item (pre-session and in File > Open) builds
  one PointCloud per scan using the embedded pose as the initial pose;
  E57 loads never write a session/result folder. "Update e57 poses"
  writes the current registered poses back, either overwriting the
  originals or as <name>_updated.e57.
- PointCloud gains non-serialized e57_source_path / e57_scan_index for the
  write-back; File > Open is also reachable once a session is loaded.

New mandeye::e57io::save_e57() writes a list of scans (borrowed point/
intensity/colour/timestamp buffers + a pose each) to a fresh multi-block
.e57 file, streaming points in 1M-point chunks. Cartesian coordinates are
written as double FloatNodes; intensity as 0..65535, colour as 0..255.

Step 2 gains a File > "Save session as e57" item: one Data3D block per
point cloud, points in scan-local frame, each block carrying
m_pose (+ session offset) as its file-level pose. Round-trips through
load_e57 (geometry, intensity, colour presence and pose verified).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>