#50968: ci: split macos-x64 tests into 3 shards
The macos-x64 / test / test (darwin, 1) shard has been hitting the step timeout at high rate since 2026-04-06 — see e.g. https://github.com/electron/electron/actions/runs/24181403207 (4 consecutive timeouts on 41-x-y). Successful runs already take ~37-43 min and recent test additions plus the MacWebContentsOcclusion re-enable pushed it past the 40-min step limit on release branches.
Rather than bump the timeout, this splits macos-x64 (darwin and mas) from 2 shards to 3, matching what linux already does. script/split-tests redistributes 77 spec files from 39+38 → 26+26+25.
Net effect: macos-x64 goes from 4 → 6 test jobs ({darwin,mas} × {1,2,3}). macos-arm64 and Windows stay at 2 shards.
Notes: none
Backports
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