Fix Epeck_d spatial searching compilation

[UtkarsHMer05]

Summary

Fixes Spatial Searching compilation failures when using Epeck_d.

Iterator_from_indices already implements the operations required by a
random-access iterator through increment(), decrement(), advance(),
and distance_to(), but advertised only bidirectional traversal.

This change:

• advertises random-access traversal;
• adds an explicit operator[] returning the iterator reference type,
  avoiding the inherited Boost proxy result;
• adds a regression test for static and dynamic Epeck_d;
• covers Fuzzy_iso_box, Fuzzy_sphere, Kd-tree insertion, search,
  removal, and size verification.

The regression test is enabled when Eigen support is available.

Fixes #9534.

Validation

Validated from a clean worktree based on public/main commit
10968626f93fca92bd0b2b934c2899819e732bb1.

• Confirmed the regression fails on the unchanged baseline with the
  expected iterator/random-access compilation errors.
• Built all 17 Spatial Searching test targets in Release.
• Built all 17 Spatial Searching test targets in Debug.
• Built all 4 NewKernel_d test targets in Release and Debug.
• Ran the new regression in Release and Debug.
• Ran assertion-enabled Epick_d, Epick_d_eigen, test_bbox_d,
  remove, and Splitters tests.
• Ran the tri2 Debug test with 1,000 points.

[mglisse]

This makes sense to me.
Note that it does not completely fix #9534: Fuzzy_iso_box.h uses an operation not guaranteed by the concept, so either that file or the concept needs to change, although that can be done in a separate PR, I don't care.