[Website] Share concurrent OPFS autosave operations

[adamziel]

Shares one in-flight autosave operation between concurrent callers for the same Playground.

Before this PR, duplicate autosaveTemporarySite() calls ran the entire persistence pipeline twice: creating the pending OPFS record, copying the filesystem, updating metadata, and pruning old autosaves. PR #4136 serializes the final metadata writes; this follow-up prevents the duplicate pipeline in the first place.

The in-flight entry is removed after success or failure, so a later call can retry normally.

Testing

Start two autosaves for one temporary Playground at the same time. Confirm both callers complete with the same stored Playground and only one persistence operation runs.

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Shares a single in-flight OPFS autosave operation across concurrent callers for the same temporary Playground, preventing duplicate persistence pipelines and consolidating routing/pruning behavior.

Changes:

• Add per-store (per-dispatch) tracking of in-flight autosaves and reuse the same promise for concurrent callers.
• Consolidate pruning exclusions and defer URL routing so it can run if any concurrent caller requests it.
• Add Vitest coverage for shared autosave behavior and finalization timing.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
packages/playground/website/src/lib/state/redux/site-management-api-middleware.ts	Deduplicates concurrent autosave work via an in-flight map and consolidates routing/pruning behavior.
packages/playground/website/src/lib/state/redux/site-management-api-middleware.spec.ts	Adds tests verifying autosave promise sharing, finalization waiting, and option aggregation.

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