fix(test): settle fake checkpoint writer so main-slice test drains cleanly

[onlyfeng]

Summary

checkpoint-main-slice test: the recording actor's spawn outcome never resolved,
leaving tryStartCheckpointWriter's settlement-watcher fiber suspended until scope
close force-interrupted it — a flaky source under it.live. Settle the outcome so
the watcher completes (writers.delete) and assert the writer drains to no-writer.

No production change — actor/spawn.ts is untouched; the test's spawn-ref finalizer
stays = undefined, consistent with the sibling checkpoint tests.

Relation to upstream

Same change set as XiaomiMiMo#1400 (fork → upstream), shared head branch.

Verification (local)

• bun typecheck — exit 0
• bun test test/session/checkpoint-main-slice.test.ts — 1 pass / 13 assertions

🤖 Generated with Claude Code

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Avoid restoring disposed spawn refs

When two Actor.layer scopes overlap but do not shut down in strict LIFO order, this can resurrect a finalized service: scope A installs implA, scope B captures it and installs implB, A finalizes first and does nothing because current !== implA, then B finalizes and restores the now-disposed implA. Consumers such as checkpoint writer spawning read spawnRef.current later, and a stale actor service can attempt to forkIn A's closed scope. Consider tracking active entries/removing this impl from a stack rather than restoring a single saved pointer.

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[onlyfeng]

Reverted the spawn.ts spawn-ref change (and the test's matching save-restore). This PR is now the test-only flaky fix; the spawn-ref finalizer is unchanged from main and stays consistent with the sibling checkpoint tests' = undefined, which removes the restore-disposed-ref path the P2 comment flagged.

[onlyfeng]

CI note — the red unit shards are pre-existing base flakiness, unrelated to this PR.

• Both failures are server live-runtime tests (workflows-route, session-prompt-heartbeat) returning HTTP 403 in CI; both pass locally (19 pass). 403 points at the CI environment (auth/network), not test logic.
• The same two tests failed identically before and after the rollback (efcf6bc → 946b80b), confirming they're unrelated to the checkpoint change. The fork base 89f212b itself has a failing unit job on main.
• Net change here is test-only (src/actor/spawn.ts untouched); all related tests pass locally.