Three changes in one commit:
1. NEW lib/util/atomic-write.mjs — exports atomicWriteJson(path, obj),
the canonical tmp+rename pattern. Reused by pre-compact-flush.mjs and
(in subsequent steps) by the new session-state writer.
2. NEW tests/lib/atomic-write.test.mjs — 4 unit tests covering
round-trip, no-orphan-tmp, overwrite-atomic, pretty-print formatting.
3. REFACTOR hooks/scripts/pre-compact-flush.mjs — replace the inline
atomicWrite() with the imported atomicWriteJson(). Also fixes a
pre-existing syntax error (leading whitespace + stray --resume token
outside the comment block) that silently broke the hook from v3.1.0
onward — PreCompact runtime is fail-open and swallowed the error.
File reformatted with standard zero-indent JS.
163 → 167 tests, 0 fail.
Step 2 of /ultracontinue v3.3.0 (project 2026-05-01-ultracontinue).
