fix: replace deprecated np.char.chararray with np.full

[xyf5432]

Fixes #1210

Problem

np.char.chararray((n, n)) is deprecated in numpy 2.5.0 and emits a DeprecationWarning on every call — the constructor runs on each invocation of do_something, producing repeated warning noise.

Fix

Use a regular numpy array filled with the initial value instead:

- grid = np.char.chararray((n, n))
- for i in range(n):
-     for j in range(n):
-         grid[i][j] = "*"
+ grid = np.full((n, n), "*", dtype="<U1")

np.full creates an identical 2D string array in one call and works on all numpy versions.

Verification

• Verified the replacement in isolation: same 2D shape, same filled content, and single-character element assignment (#, -) works identically.
• One intentional, cosmetic difference: the old chararray stored bytes (b'*'), the new array stores unicode ('*') — the display loop now prints * instead of b'*'. Grid values are never compared programmatically, so no logic depends on this.
• No deprecated API access remains.