102 task add GridCell model

[kimit0310]

This PR resolves #102

This PR introduces GridCell model and unit tests for them to assign strokes based on centroid location, used for DSYM and Alphabet tasks.

Grid cells use half-open intervals [min, max) to avoid double-assignment on shared boundaries. The last cell in each dimension is fully inclusive [min, max].

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.63%. Comparing base (da2c270) to head (f2c46d7).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #119      +/-   ##
==========================================
+ Coverage   98.57%   98.63%   +0.05%     
==========================================
  Files          19       19              
  Lines        1054     1097      +43     
==========================================
+ Hits         1039     1082      +43     
  Misses         15       15              

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[Asanto32]

Some small edge cases to think about + use of pydantic basemodel instead of dataclass?

[Asanto32]

Minor discrepancy between test docstring and actual test, otherwise lgtm