[wingleeio]

This is the guard trim deferred from #552. DisplayLink::refresh_phase runs on every steady frame: once for the latch's grid and once for the align wait before handover. Each call locked the link registry, the mutex the CoreVideo thread holds through every callback, and read the clock twice.

What changes:

• The grid, meaning the last refresh's host time and the period, moves into an Arc<Grid> of two atomics. The display's entry and the windows on it share it, and follow_display swaps it when a window changes screens.
• refresh_phase(fresh) reads it without the lock and with one mach_absolute_time. The phase is that of the moment of the call; both callers passed Instant::now() there anyway.
• Its at parameter is gone. It was crate-internal, so no public API changes.

Same behaviour: the same grid and the same phase, give or take the nanoseconds between the two clock reads it used to make. Without the lock, a reader can see a period from one callback beside a refresh time from the next. That can only matter while the display changes mode, and the comment says so.

Measured: animate, dist, 6 alternating 4 s runs each, /usr/bin/time -l.

	main	this
instructions per frame shown	3,179 K	3,182 K
fps (120 Hz panel)	116.55	117.00

Both are within run-to-run noise (about 2.5% between runs). The cut is about 100–200 instructions a frame plus one lock the link thread contends for, too small to resolve here. It removes UI-thread versus link-thread lock contention from the frame's handover path rather than buying a measurable instruction win.

Tests: cargo test -p zgx-desktop, clippy 1.99 -D warnings, fmt.

🤖 Generated with Claude Code

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[github-actions]

Instruction counts (cachegrind, exact) against 4132a64d; the gate fails above +1%.

scene	base	head	change
heavy-both	1,411.8 M	1,411.9 M	+0.00%
heavy-stream	1,355.8 M	1,355.8 M	-0.00%
heavy-scroll	80.3 M	80.3 M	-0.00%
lab-stream	462.0 M	462.0 M	-0.00%
lab-scroll	70.1 M	70.1 M	+0.00%
animate	394.8 M	394.8 M	+0.00%
micro-state	59.7 M	59.7 M	-0.01%
micro-vscroll-fixed	48.7 M	48.7 M	-0.01%
micro-vscroll-variable	89.0 M	89.1 M	+0.06%
micro-state-luau	82.9 M	82.9 M	+0.01%
micro-vscroll-fixed-luau	66.2 M	66.2 M	-0.02%
micro-vscroll-variable-luau	134.6 M	134.6 M	-0.02%
mount	327.4 M	327.4 M	+0.01%
script-update	1,698.5 M	1,698.2 M	-0.02%
anime	4,672.7 M	4,672.7 M	+0.00%
script-layout	3,244.3 M	3,243.2 M	-0.04%
list-state	893.5 M	893.7 M	+0.02%
style	2,084.3 M	2,084.3 M	+0.00%
widgets	2,684.5 M	2,684.5 M	+0.00%
app-mail	318.8 M	318.5 M	-0.09%
app-chat	53.2 M	53.2 M	+0.06%
app-dashboard	189.5 M	189.5 M	-0.03%
app-dashboard-sort	126.3 M	126.3 M	+0.01%
app-sketch	48.4 M	48.5 M	+0.08%
app-code	99.8 M	99.9 M	+0.04%
app-code-search	87.6 M	87.6 M	+0.04%