Shorten macOS CI with four test shards

[JacobStephens2]

Summary

Split the macOS suite across four runners, retaining the stable test-macos check and requiring every shard to succeed.

CI
  test                      cargo fmt, clippy, cargo test on Ubuntu
  test-macos-shard (1–4)     macos-26, nextest hash:i/4
    test-macos              succeeds only when all four shards pass

Closes #570

Evidence

• Before: CI on the exact base commit reached the macOS result in 41m 44s.
  After: This PR's CI reached the final macOS result in 19m 25s, 53.5% shorter. Both times run from CI creation through completion of test-macos; the new time includes one shard waiting over ten minutes for a runner.
• Compared every passing (binary, test) pair in the base log with all four shard logs: the same 1,462 macOS tests passed exactly once, with no missing or duplicate tests. The longest shard's test execution was 12m 36s, versus 40m 29s unsharded.
• All CI and release build checks passed. Local cargo fmt --check, cargo check --all-targets, cargo clippy --all-targets -- -D warnings, and cargo test --test toolchain passed. Full local cargo test passed: 1516 passed, 0 failed, 1 ignored across 39 test binaries.
• actionlint .github/workflows/ci.yml passed. The final gate's actual shell command accepted success and rejected failure, cancelled, skipped, and empty results. cargo nextest list --message-format json, with the workflow's four partitions, selected all 1,516 runnable Linux tests exactly once.

Test seams: nextest's selected test lists and shard commands; the workflow's final status command; the existing Rust suite.

Merge Danger

Door: two-way

Reverting the workflow returns to one macOS runner.

Blast Radius: CI

Four macOS runners replace one, with additional per-run setup and builds. Only shard 1 writes the shared cache. The final check adds an Ubuntu job after the shards finish.

Unaddressed findings

• Standards: none; 0 findings.
• Spec: none; 0 findings.

Unread changed files: none on either axis. Both reviewers read the only changed file, .github/workflows/ci.yml.

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