Retain same-turn canceled terminal storage flush through reload handoff #884

Parent acceptance epic: #269
Predecessor: #882 / PR #883

Problem

Unchanged VS Code OSS 1.137 stores MCP workspace input metadata synchronously during beforeunload, schedules Storage.flush(0), and then disposes its browser workbench in the same lifecycle dispatch. Delayer.dispose() calls clearTimeout() for that just-created zero-delay flush. WebScene removes the task, finds no terminal persistence work, dispatches pagehide immediately, and publishes the reload before the pending storage item reaches IndexedDB.

The installed exact package at AppScene cd0a02e + WebScene 2520661b reproduces the loss. The pre-reload registry contains ${input:sceneSecret}; after reload the workspace registry is empty. Awaiting the unchanged upstream IStorageService.flush(SHUTDOWN) before location.reload() makes the same package pass, isolating the defect to terminal lifecycle handoff rather than IndexedDB durability.

Reproduction

A focused native fixture now models the exact sequence:

1. beforeunload creates a zero-delay one-shot flush timer.
2. normal workbench disposal clears that timer before event dispatch returns;
3. the callback would start a read/write IndexedDB transaction and a promise/timer continuation;
4. pagehide closes the database;
5. durable restart must recover the value.

At current main the fixture fails with:

order=[beforeunload, preexisting-commit, pagehide]
committed=false
continuation=false

Proposed fix

Treat a zero-delay, non-repeating callback created while dispatching an accepted terminal lifecycle as committed terminal handoff work. If same-turn disposal clears its public timer handle, detach it from the public timer namespace but retain one private terminal callback under the existing 1,024-task handoff budget. Preserve terminal ancestry through its microtasks, timers, and IndexedDB operations. Do not retain intervals, animation frames, future timers, timers created outside terminal lifecycle, or work beyond the bounded budget.

This is a narrow native-shell durability contract needed by browser workbenches whose synchronous shutdown disposal cancels their delayed flush object. Document the deliberate terminal-only difference from ordinary clearTimeout() behavior.

Acceptance

• Focused fixture fails before and passes after the runtime change.
• beforeunload → retained flush callback → IndexedDB commit → continuation → pagehide → host reload order is exact.
• Restart recovers both the lifecycle write and a transaction already pending before reload.
• Intervals and future timers do not delay navigation.
• Self-refilling terminal work remains bounded by the existing task budget.
• Ordinary clearTimeout() outside an accepted terminal lifecycle still cancels exactly.
• Exact unchanged Code OSS package passes three sequential MCP save/reload/reopen cycles without the diagnostic explicit-flush probe.
• Record focused elapsed time and verify no idle-loop or retained-task regression.