fix(archives): serialize duplicate archive extractions

[jonathanKingston]

Concurrent requests to unpack the same ZIP delete and write the same .partial directory. One request can remove another’s in-progress files or fail during rename.

Use the existing keyed serializer around extraction and reuse, keyed by the final archive directory. Duplicate requests wait for the first extraction and reuse its completed files; different archives retain independent queues. A concurrent three-request regression verifies all callers succeed and every file survives.

Validation: pnpm run check. The concurrent regression fails on current main. Filesystem plumbing only; no DOM or layout changes.

[github-actions]

🖥️ PR preview

• Marketing site
• Browser demo
  Interactive, with a mocked backend — jump to a scenario:
• Builds a cupcake site
• Agent-proposed edits open the Changes panel
• Markdown list indentation
• Responsive composer footer
• Subagent display visual reference
• Subagent stays ungrouped beside reading tools
• Semantic search subagent markdown
• Light-theme approval with a bright accent
• Light-theme syntax, fills, and selection
• Grouped outside-sandbox command approval
• Remote desktop discovered-port list with one selected
• Settings scroll + sticky footer geometry
• Per-model reasoning / temperature / top-p
• Thread switch hydration notice
• Thread switch hydration failure
• Chat layout styling
  _{Static demo of the renderer against a mock API — the browser pane, terminal, and all main-process IPC are stubbed, so it never proves main-process behaviour. Rebuilt whenever a push changes the demo bundle; these links go live once the queued deploy lands, shortly after this comment. Removed when the PR closes.}