fix(filesystem): harden file replacement with EPERM fallback

[Chelebii]

Problem

When \s.rename\ fails with \EPERM\ on Windows (e.g. VS Code or antivirus holding a file lock), both \writeFileContent\ and \�pplyFileEdits\ propagate the error even though the write could succeed via a fallback path. Additionally, if the fallback copy succeeds but temp file cleanup fails (antivirus briefly locks the temp), the user sees an error despite the target file being written correctly.

Ref: #3430, #3199

What changed

• Extracted
  eplaceFileFromTemp\ helper from the inline rename logic in \writeFileContent\ and \�pplyFileEdits, eliminating code duplication.
• Added EPERM fallback: when \s.rename\ fails with \EPERM, falls back to \s.cp({ force: true })\ + best-effort \s.unlink\ of the temp file.
• Best-effort temp cleanup: if \s.cp\ succeeds but \s.unlink(tempPath)\ fails, the error is swallowed — the target was already written successfully.
• JSDoc documentation: documents \FILE_SHARE_DELETE\ limitation and the non-atomic nature of the \s.cp\ fallback.
• 5 new tests covering:
  • EPERM fallback through \writeFileContent\
  • EPERM fallback through \�pplyFileEdits\
  • Successful write despite temp cleanup failure (\writeFileContent)
  • Successful edit despite temp cleanup failure (\�pplyFileEdits)
  • Existing behavior preserved (non-EPERM errors still propagate)

Validation

\
npx vitest run
✓ tests/lib.test.ts (49 tests)
✓ tests/path-utils.test.ts (29 tests)
✓ tests/roots-utils.test.ts (3 tests)
✓ tests/path-validation.test.ts (53 tests)
✓ tests/directory-tree.test.ts (7 tests)
✓ tests/structured-content.test.ts (5 tests)
✓ tests/startup-validation.test.ts (4 tests)

Test Files 7 passed (7)
Tests 150 passed (150)
\\

\ sc --noEmit\ — 0 errors.

Checklist (from #3430)

• Temp file cleanup is best-effort (no false errors on successful writes)
• JSDoc documents \FILE_SHARE_DELETE\ requirement and non-atomic fallback
• \�pplyFileEdits\ has EPERM fallback test coverage
• Function name accurately reflects behavior (
  eplaceFileFromTemp)
• All acceptance tests pass

Fixes #3430