[stan69b]

Fixes #2535.

What changes

VIS_RING goes from 8 (ALLOC_MAX) to 128 in nvn_mtl.m. vis_take already grows the ring lazily and reuses a buffer once its submission's fence passes, so the ring only grows to the number of counting submissions actually in flight. That's at most 8 MiB, all kept resident.

Why it matters: since fd8188c9, a frame's submissions share a command allocator, so more counting submissions are in flight than allocators. With the GPU behind, the ring of 8 was often all in flight. Each counting submission then made a 64 KiB buffer of its own and added it to the residency set, then removed it after the submission. That's two residency commits through the kernel's GPU driver per counting submission.

Measured

iPhone 17 (A19, 8 GB, iOS 27.2.1), Ocarina of Time at the launcher's 30 fps cap with dynamic resolution, from a save. The build was the owner's local build (main 547c5db6 with #1971, #2270 and #2241), with and without this change:

	Before	After
Kakariko Village, fps	8–12	25–26
Kakariko, frame p50	62–118 ms	37 ms
Kakariko, GPU p50	25–45 ms	20 ms
Kakariko, residency commits per 300 frames	1,700–5,000	51–89
Hyrule Field, fps	10–12	29.5
Hyrule Field, frame p50	69–78 ms	33 ms

The cause was found from a sysdiagnose spindump. The game's main thread was in nvnSyncWait → mtl_wait_at, and the Metal 4 submission threads were in IOGPUFamily. Details are in the issue.

Checked

• t_wait_event: a new case runs 24 counting submissions in flight, none complete. Each takes a distinct ring buffer, and once they complete the next one reuses one without growing the ring. The case fails on main (2 of its 3 checks).
• t_render_enable_queue, t_render_counter_overlap, t_counter_clock and test_docs pass.
• The Mac host builds.
• Docs: a reference entry in docs/reference/frame-rate.md after the shared-allocator entry, the symptom index, and a line under Ocarina of Time's "Found on this game".

Regression check

Not run yet. On this Mac only Ocarina of Time reaches counting submissions often enough to matter, and a Mac never ran the ring dry (the reference entry for fd8188c9 says the same for an M4 Pro). A scripts/regress.py check on Ocarina of Time will be added here when it has run.