Initial firmware for ESP32-S3 six-channel soil moisture node

Implements ESP32_Soil_Moisture_Node_PRD.md on the VCC-GND Studio
YD-ESP32-S3 (ESP32-S3-WROOM-1) using Arduino via PlatformIO, publishing
to Home Assistant over MQTT with auto-discovery.

Two power profiles behind a compile-time switch (PRD 4):
  -e mains    always on, publishes every 60s
  -e battery  RTC wake, sample, publish, deep sleep 15min

PRD mitigations:
  - All six probes on ADC1 (GPIO4-9); ADC2 is unusable while Wi-Fi is up.
  - analogReadMilliVolts() for eFuse ADC calibration rather than raw counts.
  - 16 samples per probe, sorted, 10% trimmed from each end, averaged.
  - Per-probe two-point calibration; HW-390 output is inverted (dry reads
    high), and an inverted config is rejected rather than silently producing
    a backwards curve.
  - MAC-derived node ID so identical binaries never collide on topics.

Design decisions that diverge from a literal reading of the PRD:
  - Out-of-range probes publish null (HA "unknown") instead of clamping to
    0%/100%. A cut Cat5 pair reads ~0mV, which clamps to a believable "very
    dry" and would drive irrigation on wrong data.
  - The battery profile uses expire_after rather than an availability topic;
    a sleeping node is offline most of the time and would sit greyed out.
  - AWAKE_BUDGET_MS forces deep sleep if a wake cycle overruns, covering
    failure modes the per-step network timeouts miss (DHCP never issued,
    CONNACK never sent) that would otherwise flatten the cell.
  - docs/WIRING.md notes the 18650 cannot feed the probes directly: the
    XC6206 LDO needs ~3.4V headroom, so a boost converter is required on the
    switched side of the MOSFET.

Both profiles compile clean under -Wall. Not yet run on hardware; pin map,
MOSFET circuit and calibration defaults are unverified against a board.

Co-Authored-By: Claude <noreply@anthropic.com>

platformio.ini

@@ -0,0 +1,52 @@
+; ESP32-S3 6-channel soil moisture node
+; Board: VCC-GND Studio YD-ESP32-S3 (ESP32-S3-DevKitC-1 clone, ESP32-S3-WROOM-1)
+;
+; Two power profiles, selected by which environment you build:
+;   pio run -e mains    -t upload    ; always-on, 5V USB / buck
+;   pio run -e battery  -t upload    ; 18650 + TP4056 + solar, deep sleep
+
+[platformio]
+default_envs = mains
+
+[env]
+platform = espressif32@^6.5.0
+board = esp32-s3-devkitc-1
+framework = arduino
+
+; YD-ESP32-S3 ships in several flash/PSRAM variants (N8, N8R2, N8R8, N16R8).
+; These flags target the common N16R8 (16MB flash, 8MB octal PSRAM).
+; For an N8 board with no PSRAM, drop the two BOARD_HAS_PSRAM/OPI lines
+; and set board_upload.flash_size / board_build.partitions accordingly.
+board_build.flash_mode = qio
+board_build.partitions = default_16MB.csv
+board_upload.flash_size = 16MB
+
+monitor_speed = 115200
+monitor_filters = esp32_exception_decoder, time
+
+lib_deps =
+    knolleary/PubSubClient@^2.8
+    bblanchon/ArduinoJson@^7.0.4
+
+build_flags =
+    -DARDUINO_USB_MODE=1
+    -DARDUINO_USB_CDC_ON_BOOT=1
+    -DBOARD_HAS_PSRAM
+    -DCORE_DEBUG_LEVEL=3
+    -Wall
+
+; ---------------------------------------------------------------------------
+; Continuous power: MCU stays awake, publishes on a fixed interval.
+; ---------------------------------------------------------------------------
+[env:mains]
+build_flags =
+    ${env.build_flags}
+    -DPOWER_PROFILE_MAINS
+
+; ---------------------------------------------------------------------------
+; Solar / battery: RTC-timer wake, sample, publish, deep sleep.
+; ---------------------------------------------------------------------------
+[env:battery]
+build_flags =
+    ${env.build_flags}
+    -DPOWER_PROFILE_BATTERY