Add SD logging and update docs

- Add optional microSD CSV logging per sender/day on receiver
- Wire logger into receiver packet handling
- Document new batch header fields, build envs, and SD logging
- Make sender links open in a new tab

src/sd_logger.cpp

@@ -0,0 +1,114 @@
+#include "sd_logger.h"
+#include "config.h"
+#include <SD.h>
+#include <SPI.h>
+#include <time.h>
+
+static bool g_sd_ready = false;
+
+static const char *fault_text(FaultType fault) {
+  switch (fault) {
+    case FaultType::MeterRead:
+      return "meter";
+    case FaultType::Decode:
+      return "decode";
+    case FaultType::LoraTx:
+      return "loratx";
+    default:
+      return "";
+  }
+}
+
+static bool ensure_dir(const String &path) {
+  if (SD.exists(path)) {
+    return true;
+  }
+  return SD.mkdir(path);
+}
+
+static String format_date_utc(uint32_t ts_utc) {
+  time_t t = static_cast<time_t>(ts_utc);
+  struct tm tm_utc;
+  gmtime_r(&t, &tm_utc);
+  char buf[16];
+  snprintf(buf, sizeof(buf), "%04d-%02d-%02d",
+           tm_utc.tm_year + 1900,
+           tm_utc.tm_mon + 1,
+           tm_utc.tm_mday);
+  return String(buf);
+}
+
+void sd_logger_init() {
+  if (!ENABLE_SD_LOGGING) {
+    g_sd_ready = false;
+    return;
+  }
+  g_sd_ready = SD.begin(PIN_SD_CS);
+}
+
+bool sd_logger_is_ready() {
+  return g_sd_ready;
+}
+
+void sd_logger_log_sample(const MeterData &data, bool include_error_text) {
+  if (!g_sd_ready || data.ts_utc == 0) {
+    return;
+  }
+
+  String root_dir = "/dd3";
+  if (!ensure_dir(root_dir)) {
+    return;
+  }
+
+  String sender_dir = root_dir + "/" + String(data.device_id);
+  if (!ensure_dir(sender_dir)) {
+    return;
+  }
+
+  String filename = sender_dir + "/" + format_date_utc(data.ts_utc) + ".csv";
+  bool new_file = !SD.exists(filename);
+  File f = SD.open(filename, FILE_APPEND);
+  if (!f) {
+    return;
+  }
+
+  if (new_file) {
+    f.println("ts_utc,p_w,p1_w,p2_w,p3_w,e_kwh,bat_v,bat_pct,rssi,snr,err_m,err_d,err_tx,err_last");
+  }
+
+  f.print(data.ts_utc);
+  f.print(',');
+  f.print(data.total_power_w, 1);
+  f.print(',');
+  f.print(data.phase_power_w[0], 1);
+  f.print(',');
+  f.print(data.phase_power_w[1], 1);
+  f.print(',');
+  f.print(data.phase_power_w[2], 1);
+  f.print(',');
+  f.print(data.energy_total_kwh, 3);
+  f.print(',');
+  f.print(data.battery_voltage_v, 2);
+  f.print(',');
+  f.print(data.battery_percent);
+  f.print(',');
+  f.print(data.link_rssi_dbm);
+  f.print(',');
+  if (isnan(data.link_snr_db)) {
+    f.print("");
+  } else {
+    f.print(data.link_snr_db, 1);
+  }
+  f.print(',');
+  f.print(data.err_meter_read);
+  f.print(',');
+  f.print(data.err_decode);
+  f.print(',');
+  f.print(data.err_lora_tx);
+  f.print(',');
+  if (include_error_text && data.last_error != FaultType::None) {
+    f.print(fault_text(data.last_error));
+  }
+  f.println();
+  f.close();
+}