Tray slider drag mid-zoom resets magnifier.ini to defaults (stuck zoom, setup at next start) #377

Field 2026-10-07: while zoomed, dragging the tray's Night light slider left the zoom stuck at its level (zoom keys stopped working), and restarting Wind opened the first-launch setup. All settings were reset to factory defaults.

Cause. Writers replace magnifier.ini atomically (MoveFileExW replace), but around each replace the name briefly refuses opens. Measured on the dev PC: about 1% of reads during a burst of replaces failed with ERROR_ACCESS_DENIED. LoadConfig treated a failed open as a missing file and wrote the defaults over the user's ini. Read-modify-write callers using ReadTextFile got "" on a failed read.

Evidence (wind-core.log). 19:36:04-07Z: colour effect applied several times (slider drag), then bitmap smoothing 0 (a defaults reload; the ini had 1). After that, raw X-button events arrive but no inHeld/outHeld: zoom buttons were unbound. On restart, onboarded=0 opened the setup.

Repro. Hold the ini open with no sharing for 1.5 s right after changing it (C:\RE harness ini_lock_test.py). Unfixed develop: zoom dead afterwards.