Clean up dead code: unused GRID_SZ in PrefixSumExecutor.__init__

[alanray-tech]

Description

In python/quadrants/algorithms/_algorithms.py line 58, GRID_SZ is computed but never used:

BLOCK_SZ = 64
GRID_SZ = int((length + BLOCK_SZ - 1) / BLOCK_SZ)

GRID_SZ appears only once in the entire file — it is assigned but never referenced afterward. This is likely leftover from a refactor.

Suggested fix

Remove the unused GRID_SZ assignment on line 58.

Found during

Review of PR #642.