Address safety review findings for the dual-interface (WiFi + USB serial)
architecture running on the ESP32-S3's two Xtensa LX7 cores:
- Protect sweep state with std::atomic (acquire/release ordering)
- Add Attenuator::getSnapshot() for consistent multi-field reads
- Add advanceStep()/persistCurrent() to eliminate TOCTOU races
- Switch to StaticSemaphore_t (compile-time mutex, can't fail)
- Accumulate web server POST bodies before parsing (chunked TCP fix)
- Backport USB serial input validation to web server handlers
- Auto-stop sweep on manual set (prevents silent overwrite)
- Validate WiFi TX power against known-good levels
- Add OTA password authentication support
- Check NVS write return values, log failures
- Reset USB serial buffer on reconnect (stale overflow fix)
- Rename sweep.h to app.h (declares more than sweep functions)
Ryan Malloy
4e19882d32