Eliminate portrait eighth-snap gaps on displays whose height is not divisible by 4

[YuriNachos]

Summary

Portrait-mode eighth snapping abuts cleanly: the middle row seams that previously left 1-2px gaps on displays whose height is not divisible by 4 now tile exactly. The unavoidable floor(height/4) remainder collects at the bottom edge instead of being split across the middle seams.

Root cause

The four portrait eighth calculations set each row's height to floor(visibleFrame.height / 4) but derived two row origins from raw screen fractions (height * 0.75, height / 2.0). When height % 4 != 0, the raw-fraction boundaries did not land on the floored cell edges, so the middle seams (rows 1-2 and 2-3) left 1-2px gaps.

Changes

• Rectangle/WindowCalculation/{TopCenterRightEighth,TopRightEighth,BottomLeftEighth,BottomCenterLeftEighth}Calculation.swift — express each of those two row origins as a cell-height multiple (rect.height * 2, rect.height * 3) so the top three rows share one consistent floor(height/4) basis and abut exactly against each other and against the top-anchored row 1.
• RectangleTests/RectangleTests.swift — a PortraitEighthAbutmentTests class: for a non-divisible height (1002) it asserts the row origins form the maxY - cellH*k grid and the two middle seams abut with no gap/overlap; for a divisible height (1000) it asserts all three seams abut (control).

Behavior notes

• For heights divisible by 4 the math is numerically identical to before (floor(h/4)*3 == h*0.75, floor(h/4)*2 == h/2), so the already-seamless case is unchanged.
• For non-divisible heights the floor remainder is no longer scattered across the two middle seams; it collects as a single 1-2px step at the bottom edge (between the third row, anchored from maxY, and the bottom row, anchored at minY). This matches Rectangle's existing top/bottom anchoring convention; the test documents it explicitly. (A fully-seamless tile would require the bottom row to absorb the remainder with a non-uniform height, which is a separate design choice.)

Testing

xcodebuild test -project Rectangle.xcodeproj -scheme Rectangle -destination 'platform=macOS' — 208 tests, 21 failures (0 unexpected). The 21 are the pre-existing red baseline on main (cooperative-resize / corner-calculation suites, issues #1804–#1806); this change adds zero new failures and the new abutment tests pass.

AI assistance

This change was developed with AI assistance (Claude Code); every changed line was reviewed and understood by the contributor.