Rewrite find_enclosing_cells using segment-face intersections

[strene]

Replace the point-sampling approach with exact intersection of each trajectory segment against the mesh faces. Faces are triangulated as a fan around the face centroid (watertight between neighboring cells) and a uniform bucket grid limits the faces tested per segment. The crossing direction at each intersection determines which cell is entered, so each part of the trajectory between consecutive crossings is assigned to a single cell. Multiple crossings of the same non-planar face are handled.

• Cells are returned in order of first visit, as before.
• extra_out lengths are now exact, and a new :centroids entry gives the length-weighted centroid of the subsegments in each cell.
• Fix :normed_direction using the list index instead of the cell index in cell_dims.
• n and limit_box are kept for backwards compatibility but no longer have an effect.
• Roughly 100x faster than the previous version on a 500k-cell mesh.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 93.75000% with 19 lines in your changes missing coverage. Please review.
✅ Project coverage is 57.13%. Comparing base (6616b55) to head (5c12efd).

Files with missing lines	Patch %	Lines
src/meshes/trajectories.jl	93.75%	19 Missing ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #371      +/-   ##
==========================================
- Coverage   57.22%   57.13%   -0.09%     
==========================================
  Files         222      211      -11     
  Lines       29847    29655     -192     
==========================================
- Hits        17079    16944     -135     
+ Misses      12768    12711      -57     

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