Stabilized logging to SD Card, disciplined time, and web fetch & erase

exercises/18_GPS_Field_QA/README.md

@@ -2,11 +2,11 @@
 
 Survey/reconnaissance firmware for LilyGO T-Beam SUPREME.
 
-This exercise measures GNSS visibility and solution quality, logs results to internal flash using CSV, and provides a minimal serial interface for retrieving the logs in the field.
+This exercise measures GNSS visibility and solution quality, disciplines the RTC from GNSS before creating any capture log, writes CSV captures to SD card, and exposes the SD tree over the field AP for download and erase operations.
 
 Current storage choice:
 
-- `SPIFFS`
+- `SD`
 
 Current environments:
 
@@ -18,13 +18,21 @@ Primary serial commands:
 - `status`
 - `summary`
 - `ls`
-- `cat <filename>`
+- `cat <path>`
+- `erase <path>`
 - `stop`
+- `start`
+- `flush`
+- `discipline`
 - `erase_logs`
 
 Notes:
 
+- Default environment is `cy`.
+- No log file is created until GNSS UTC plus PPS has disciplined the RTC.
+- The capture file naming format is `YYYYMMDD_HHMMSS_<BOARD>.csv`.
 - Samples are aggregated once per second.
-- Records are flushed to flash every 10 seconds.
+- Records are double-buffered in RAM and flushed to SD every 10 seconds.
 - Satellite snapshot records are written as additional CSV lines when GSV data is available.
+- The web UI exposes SD download links and `/cmd?...` actions such as `erase=/logs/20260406_093912_CY.csv`.
 - The implementation uses common NMEA parsing so it can normalize L76K and MAX-M10S output without adding a new GNSS dependency.