Stabilize source scroll synchronization test

[ashtree74]

Summary

• derive the test scroll target from the laid-out text line instead of an arbitrary clip-view pixel
• assert the exact synchronized logical line within layout tolerance
• keep production behavior unchanged

Verification

• affected test: 10 consecutive passes
• swift test: 135 XCTest + 5 Swift Testing tests passed
• python3 -m unittest discover -s site -p 'test_*.py': 9 passed\n- shell syntax and property-list validation passed\n\n## Why\n\nThe post-merge main run exposed a timing/layout-sensitive failure that had passed on the PR. This makes the test use the same geometry-backed scroll path as the existing wrapped-line synchronization test.\n\nSigned-off-by: Adam Jesionkiewicz adam@jesion.pl

[ashtree74]

Self-review completed. No blocking findings.

The patch is test-only and replaces an arbitrary pixel scroll with a target derived from TextKit layout, matching the production synchronization coordinate model. The assertion is stronger (logical line 30 ±1), the affected test passed 10 consecutive runs, and the full local suites passed.

I cannot approve my own PR, so after required checks pass I will use the repository’s disclosed maintainer PR-only bypass for the one-approval rule.