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

README.md

@@ -175,7 +175,7 @@ Packet layout:
 
 LoRa radio settings:
 - Frequency: **433 MHz** or **868 MHz** (set by build env via `LORA_FREQUENCY_HZ`)
-- SF11, BW 125 kHz, CR 4/5, CRC on, Sync Word 0x34
+- SF12, BW 125 kHz, CR 4/5, CRC on, Sync Word 0x34
 
 ## Data Format
 MeterData JSON (sender + MQTT):
@@ -205,6 +205,10 @@ Fixed header (little-endian):
 - `dt_s` u8 (seconds, >0)
 - `n` u8 (sample count, <=30)
 - `battery_mV` u16
+- `err_m` u8 (meter read failures, sender-side counter)
+- `err_d` u8 (decode failures, sender-side counter)
+- `err_tx` u8 (LoRa TX failures, sender-side counter)
+- `err_last` u8 (last error code: 0=None, 1=MeterRead, 2=Decode, 3=LoraTx)
 
 Body:
 - `E0` u32 (absolute energy in Wh)
@@ -219,6 +223,7 @@ Body:
 Notes:
 - Receiver reconstructs timestamps from `t_last` and `dt_s`.
 - Total power is computed on receiver as `p1 + p2 + p3`.
+- Sender error counters are carried in the batch header and applied to all samples.
 
 ## Device IDs
 - Derived from WiFi STA MAC.
@@ -250,6 +255,7 @@ inline constexpr uint16_t EXPECTED_SENDER_IDS[NUM_SENDERS] = { 0xF19C };
   - `/`: status overview
   - `/wifi`: WiFi/MQTT/NTP config (AP and STA)
   - `/sender/<device_id>`: per-sender details
+- Sender IDs on `/` are clickable (open sender page in a new tab).
 
 ## MQTT
 - Topic: `smartmeter/<deviceId>/state`
@@ -272,6 +278,8 @@ inline constexpr uint16_t EXPECTED_SENDER_IDS[NUM_SENDERS] = { 0xF19C };
 - `lilygo-t3-v1-6-1-test`: test build with `ENABLE_TEST_MODE`
 - `lilygo-t3-v1-6-1-868`: production build for 868 MHz modules
 - `lilygo-t3-v1-6-1-868-test`: test build for 868 MHz modules
+- `lilygo-t3-v1-6-1-payload-test`: build with `PAYLOAD_CODEC_TEST`
+- `lilygo-t3-v1-6-1-868-payload-test`: 868 MHz build with `PAYLOAD_CODEC_TEST`
 
 ## Config Knobs
 Key timing settings in `include/config.h`:
@@ -283,6 +291,7 @@ Key timing settings in `include/config.h`:
  - `BATCH_RETRY_POLICY` (keep or drop on retry exhaustion)
  - `SERIAL_DEBUG_MODE` / `SERIAL_DEBUG_DUMP_JSON`
  - `LORA_SEND_BYPASS` (debug only)
+ - `ENABLE_SD_LOGGING` / `PIN_SD_CS`
 
 ## Limits & Known Constraints
 - **Compression**: MeterData uses lightweight RLE (good for JSON but not optimal).
@@ -292,6 +301,14 @@ Key timing settings in `include/config.h`:
 - **OLED**: no hardware reset line is used (matches working reference).
 - **Batch ACKs**: sender waits for ACK after a batch and retries up to `BATCH_MAX_RETRIES` with `BATCH_ACK_TIMEOUT_MS` between attempts.
 
+## SD Logging (Receiver)
+Optional CSV logging to microSD (FAT32) when `ENABLE_SD_LOGGING = true`.
+
+- Path: `/dd3/<device_id>/YYYY-MM-DD.csv`
+- Columns:
+  `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`
+- `err_last` is written as text (`meter`, `decode`, `loratx`) only on the last sample of a batch that reports an error.
+
 ## Files & Modules
 - `include/config.h`, `src/config.cpp`: pins, radio settings, sender IDs
 - `include/data_model.h`, `src/data_model.cpp`: MeterData + ID init