fix(ssg): copy the live output folder under the directory lock for --incremental

[smiggleworth]

Summary

askr ssg --incremental copied the live output folder into its staging folder before taking the directory lock; only the final swap ran under it. Two concurrent incremental builds could race: on Windows a rename fails with EPERM while the other build is copying, and on any OS a build could stage from a half-swapped folder.

The copy now runs under the same lock as the swap through a new copyTargetIntoStage helper. The existence check happens inside the lock too, so a target that is momentarily absent mid-swap is never mistaken for a missing output folder. The lock is held only for the copy, not for site generation, so builds are not serialised for their whole duration. Each build still publishes a complete tree, so two concurrent runs both succeed and the last swap wins, and neither publishes a partial tree.

Tests

• A deterministic race test holds the lock with the target half-swapped (removed, new content pending) and asserts the staged copy waits, then contains the complete swapped tree. It failed before the change (the helper did not exist, and the old code copied immediately).
• A test covers the no-target case returning false with an empty stage.

npm run lint, typecheck, test:coverage (288 tests), test:changelog, build and test:publint pass.

Closes #155