fix(include): serialize config writes

[t8y2]

Summary

• serialize debounced Include config writes
• keep later writes progressing after an earlier failure
• flush the latest pending write during Include teardown
• log asynchronous write failures and surface them from teardown

Why

Include writes use a shared <config>.tmp path. Once one debounced write has started, another write() call can start a second _writeFile() concurrently. The two operations can race on the temporary file, causing one rename to fail with ENOENT and leaving the rejection unobserved. Teardown also returned without waiting for the last pending write.

Validation

• yarn lint
• yarn build core
• yarn build
• yarn test:json — 20 files, 167 tests passed
• real-file reproduction with overlapping public write() calls