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/telemetry_client.h

@@ -0,0 +1,36 @@
+// HTTP client for POSTing sensor readings to the Quantumancy backend's
+// `POST /api/device/telemetry` endpoint (see the design spec's "Contract"
+// section: docs/superpowers/specs/2026-07-23-esp32-sensor-node-design.md).
+//
+// UNVERIFIED AGAINST REAL HARDWARE — see README.md.
+
+#pragma once
+
+#include <stddef.h>
+#include "esp_err.h"
+#include "sensor_driver.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+// Builds the request body:
+//   {"readings": [{"sensor_type": str, "value": number, "unit": str,
+//                  "metadata": {}}, ...]}
+// and POSTs it to "<DEVICE_BACKEND_BASE_URL>/api/device/telemetry" with
+// `Authorization: Bearer <DEVICE_PAIRING_TOKEN>` and
+// `Content-Type: application/json`. Takes ownership of any non-NULL
+// `metadata` cJSON pointers in `readings` — they are freed as part of
+// building/serializing the body regardless of whether the POST succeeds.
+// No-ops (returns ESP_OK) if `count` is 0.
+esp_err_t telemetry_client_post_readings(const sensor_reading_t *readings, size_t count);
+
+// Starts the FreeRTOS task that, on a DEVICE_REPORT_INTERVAL_SEC cadence,
+// collects a fresh batch of readings from the sensor registry and POSTs
+// them via telemetry_client_post_readings(). Call once from app_main()
+// after wifi_manager_start() and sensor_registry_init_all().
+void telemetry_client_start_task(void);
+
+#ifdef __cplusplus
+}
+#endif