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/bme280.h

@@ -0,0 +1,57 @@
+// Bosch BME280 driver (temperature / humidity / pressure) over I2C.
+//
+// Implements the sensor_driver_t interface (see sensor_driver.h) so the
+// registry can init/read it generically. Register map and compensation
+// formulas are transcribed from Bosch's public BME280 datasheet (Rev 1.23,
+// section 4.2.3 "Compensation formulas", double-precision reference
+// implementation) — publicly documented, well-trodden territory, not
+// guesswork. What IS unverified: this has never talked to a real BME280 —
+// see README.md's honesty-policy section.
+//
+// Default wiring (override via `idf.py menuconfig` or by editing the pin
+// macros below before building) — see README.md's wiring section for the
+// full pinout table:
+//   BME280 SDA -> GPIO8   (BME280_I2C_SDA_GPIO)
+//   BME280 SCL -> GPIO9   (BME280_I2C_SCL_GPIO)
+//   BME280 VCC -> 3V3, GND -> GND
+//   BME280 CSB -> VCC (selects I2C mode, not SPI)
+//   BME280 SDO -> GND selects address 0x76 (default assumed here); tie SDO
+//                 to VCC instead for address 0x77 and change
+//                 BME280_I2C_ADDR accordingly.
+
+#pragma once
+
+#include "esp_err.h"
+#include "sensor_driver.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#ifndef BME280_I2C_PORT
+#define BME280_I2C_PORT      0
+#endif
+
+#ifndef BME280_I2C_SDA_GPIO
+#define BME280_I2C_SDA_GPIO  8
+#endif
+
+#ifndef BME280_I2C_SCL_GPIO
+#define BME280_I2C_SCL_GPIO  9
+#endif
+
+#ifndef BME280_I2C_ADDR
+#define BME280_I2C_ADDR      0x76 // 0x77 if SDO is tied to VCC instead of GND
+#endif
+
+#ifndef BME280_I2C_CLK_HZ
+#define BME280_I2C_CLK_HZ    100000 // 100kHz standard mode; BME280 supports up to 3.4MHz
+#endif
+
+// sensor_driver_t-compatible entry points.
+esp_err_t bme280_init(void);
+esp_err_t bme280_read(sensor_reading_t *out, size_t max_out, size_t *out_count);
+
+#ifdef __cplusplus
+}
+#endif