Fix Exodus encode treating every face as full width

[cmdupuis3]

Closes #1753

Overview

_encode_exodus finds the end of each face with np.where(row == -1), but connectivity padding is stored as INT_FILL_VALUE. The comparison never matches, so every face is counted at the maximum width. Two things follow: a mixed mesh collapses into a single element block carrying the wrong element type (triangles and quads all written as SHELL4), and the padding itself is written out as INT_FILL_VALUE + 1 in a slot where a node index belongs.

The fix matches INT_FILL_VALUE. Element blocks then split by face size, which is what Exodus requires — blocks are homogeneous.

One note for reviewers: because blocks now genuinely split, a mixed mesh is regrouped by face size on write, and nothing yet records that permutation, so the mesh reloads reordered. Two existing round-trip tests therefore compare the Exodus faces as a set in this PR instead of element-wise — test_mixed_exodus (on the mixed.exo fixture) and test_grid_ugrid_exodus_roundtrip (on outRLL1deg, which has polar triangles among its quads). The follow-up (cmd/exodus_block_reorder) records the ordering and restores both strict comparisons. This PR is green on its own, but it does temporarily loosen those two assertions.

Expected Usage

import uxarray as ux

grid_path = "/path/to/grid.nc"
data_path = "/path/to/data.nc"

uxds = ux.open_dataset(grid_path, data_path)

# this is how you use this function
some_output = uxds.some_function()

# this is another way to use this function
other_output = uxds.some_function(some_param = True)

PR Checklist

General

• An issue is created and linked
• Added appropriate labels (if your uxarray repo permissions allow it)
• Filled out Overview and Expected Usage (if applicable) sections

Testing & Benchmarking

• There is adequate test coverage of changes from this PR (add new tests if needed)

Documentation and Examples

• Docstrings updated with any function changes, and included in all new functions
• User (public) functions added to docs/api.rst; internal (private) function names start with an underscore (_)

AI Disclosure

AI Usage: Claude Opus 5

• I have tested and take responsibility for all AI-generated content in my PR.