fix: draw grid cells at exact-period positions

Renderer half of the world-true grid fix: cell k lands at
round(k * period_px_exact) instead of k * rounded-period, so the grid
no longer slides relative to windows when panning at fractional zooms.
Verified in the nested harness at zoom 0.8 (period 422.4): the gap
line's offset from a window edge is pixel-identical across 7 pan
positions spanning more than a full period.

Co-Authored-By: Claude Fable 5 <[email protected]>

 src/server/output.rs | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/src/server/output.rs b/src/server/output.rs
index 88b05bc..83bd62b 100644
--- a/src/server/output.rs
+++ b/src/server/output.rs
@@ -795,10 +795,15 @@ impl Output {
                         } else {
                             0
                         };
+                        // Cell positions from the EXACT period, rounded per
+                        // cell: a rounded-period spacing drifts from the
+                        // world-anchored windows at fractional zooms (the
+                        // grid visibly slides against window edges when
+                        // panning).
                         for col in 0..=cells.cols {
-                            let rel_x = col * frame.period_px;
+                            let rel_x = (col as f64 * frame.period_px_exact).round() as i32;
                             for row in 0..=cells.rows {
-                                let rel_y = row * frame.period_px;
+                                let rel_y = (row as f64 * frame.period_px_exact).round() as i32;
                                 get_rect(cells.cell_px, cells.cell_px, cells.color.0.as_ptr(), rel_x, rel_y, cells.corner_radius_px, inset_scaled);
                             }
                         }