feat(firmware): ESP32-P4 sensor node — Workstream I core skeleton

New firmware/esp32p4-sensor-node/ ESP-IDF (C, FreeRTOS) project skeleton
per docs/superpowers/specs/2026-07-23-esp32-sensor-node-design.md's
Workstream I:

- Wi-Fi station-mode connect with exponential-backoff reconnect
  (wifi_manager.c), credentials from a gitignored main/device_config.h
  the seeker fills in (template: device_config.h.example).
- Telemetry HTTP client (telemetry_client.c) POSTing the spec's exact
  contract shape to /api/device/telemetry with a Bearer token, via
  esp_http_client + cJSON.
- BME280 I2C driver (bme280.c) with Bosch's public double-precision
  compensation formulas, using ESP-IDF's newer driver/i2c_master.h API.
- LD2410 mmWave presence driver (ld2410.c) over UART, chosen over a
  plain PIR for its distance/motion data richness — its frame-offset
  parsing is flagged as the least-certain code in the firmware.
- sensor_driver_t registry (sensor_driver.h, sensor_registry.c) so new
  sensors are a new driver file + one array line, no main-loop changes.
- README.md: build steps, manual-config walkthrough, wiring/pinouts,
  and an explicit "what's verified vs. not" section plus a real
  hardware caveat (ESP32-P4 has no integrated Wi-Fi radio).

UNVERIFIED AGAINST REAL HARDWARE per the spec's honesty-policy note —
no ESP-IDF toolchain or physical boards available in this environment.
Syntax-checked with gcc against hand-written ESP-IDF API stubs (not
committed) as a best-effort substitute for a real idf.py build.

Workstream J (RTL-SDR experimental module) is explicitly out of scope
here; firmware/esp32p4-sensor-node/components/ is left in place for it.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

firmware/esp32p4-sensor-node/main/wifi_manager.h

@@ -0,0 +1,44 @@
+// Wi-Fi station-mode connection management, using the credentials in
+// device_config.h.
+//
+// IMPORTANT hardware caveat (honesty note, not code): the ESP32-P4 SoC has
+// no integrated 2.4GHz radio -- real Wi-Fi on P4 hardware requires a
+// companion chip (e.g. ESP32-C6) wired via SDIO/SPI running "esp-hosted",
+// which presents this exact same esp_wifi/esp_netif API to application code.
+// This file is written against that standard API surface, so it should be
+// portable unchanged to a hosted-mode P4 board or to a Wi-Fi-native target
+// (e.g. `idf.py set-target esp32s3`) -- only sdkconfig / board wiring
+// differs, not this code. See README.md for the full explanation. This is
+// exactly the kind of thing nobody can verify without the real board, so
+// it's called out explicitly rather than glossed over.
+
+#pragma once
+
+#include <stdbool.h>
+#include "esp_err.h"
+#include "freertos/FreeRTOS.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+// Initializes NVS (required by esp_wifi), the default netif, the event
+// loop, and esp_wifi in station mode, then starts connecting using
+// DEVICE_WIFI_SSID / DEVICE_WIFI_PASSWORD from device_config.h. Registers
+// an internal event handler that retries on disconnect with a capped
+// backoff. Call once from app_main() before starting the telemetry task.
+esp_err_t wifi_manager_start(void);
+
+// Blocks (efficiently, via an internal FreeRTOS event group) until the
+// station has an IP address, or until `timeout_ticks` elapses.
+// Returns ESP_OK once connected, ESP_ERR_TIMEOUT otherwise.
+esp_err_t wifi_manager_wait_connected(TickType_t timeout_ticks);
+
+// True if the station currently holds an IP (i.e. the last known state is
+// "connected"), for callers that want a non-blocking check (e.g. the
+// telemetry task deciding whether to bother building a request this cycle).
+bool wifi_manager_is_connected(void);
+
+#ifdef __cplusplus
+}
+#endif