**Problem**: writeConfigFileLocked() accessed c.tx field without synchronization
- Function reads c.tx to check if transaction is active (line 109)
- c.tx modified by begin/endTransaction under lock, but read without lock
- Race condition: c.tx could change between check and use
**Impact**:
- Inconsistent transaction handling
- File could be written directly when it should be staged
- Or staged when it should be written directly
- Data corruption risk during config imports
**Fix** (lines 108-128):
- Added documentation that caller MUST hold c.mu lock
- Read c.tx into local variable tx while lock is held
- Use local copy for transaction check
- Safe because all callers hold c.mu when calling writeConfigFileLocked
- Transaction field only modified while holding c.mu in begin/endTransaction
This maintains the existing contract (callers hold lock) while making the transaction read safe and explicit.
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