FIX: Zero cosmask at its center

[sapunyangkut]

Summary

• set the geometric center of cosmask to zero
• add a direct mask regression test
• add a single-pixel eccentricity regression for both Python and numba engines

Root cause

At the geometric center, theta_mask evaluates arctan2(0, 0) as zero. The previous cos(2 * theta) calculation therefore contributed 1 at the center, even though the eccentricity calculation expects that undefined angular contribution to be zero. For a center-only feature, this produced an eccentricity of 1000000.0 instead of 0.0.

Fixes #728.

Validation

Fresh publication checks on upstream commit 186fa02f048ef0902b6f9804874b701345a22398:

• reproduced the center value as 1.0 before the patch
• verified the center is 0.0 after the patch and every non-center mask value is unchanged
• syntax compilation and git diff --check passed

Archived test evidence from the same upstream commit, with its manifest reverified before publication:

• focused regression tests: 5 passed
• adjacent mask tests: 8 passed
• adjacent feature/refine tests: 64 passed
• broader non-slow suite: 958 passed and 95 skipped, with one known failure because the historical reproducibility_v0.4.npz fixture retains 14 previous eccentricity values

Automation disclosure

This pull request was prepared and submitted by an OpenAI Codex agent operating on behalf of @sapunyangkut. The agent reproduced the bug, applied the minimal patch, verified the test evidence, and checked the repository's public contribution rules before submission.