Run explicit formatting in the background

[renkun-ken]

Full-document and explicit-selection formatting currently block the server's event loop. Run document, range, and multiple-range formatting in a lazy background worker so other editor requests remain responsive.

• Snapshot document content and formatting options for each request; preserve custom styles and literate-document boundaries.
• Cancel work on client cancellation or document changes, and reject stale results after changes, closure, removal, or reopening.
• Retire cancelled workers and recover for subsequent formatting requests. Keep on-type formatting on its direct path.

The first explicit formatting request includes worker startup time. Custom formatter side effects now occur in the worker.

Validation: installed this branch independently and passed 45 formatting, handler, server-lifecycle, and LSP test cases with 352 assertions. Tests exercise all three request types, changed style options, disk saves, stale results, and live responsiveness/cancellation during a deliberately stalled formatter. Whitespace checks pass.

This PR contains only background formatting and its tests, based directly on master. Supersedes the formatting portion of #762.

Closes #251.