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/device_config.h.example

@@ -0,0 +1,42 @@
+// Quantumancy sensor node — per-device configuration TEMPLATE.
+//
+// HOW TO USE THIS FILE (there is no provisioning UI in this build — see
+// README.md's "Manual configuration" section for the full walkthrough):
+//
+//   1. Copy this file to `device_config.h` in this same directory:
+//        cp device_config.h.example device_config.h
+//   2. Edit device_config.h and fill in YOUR OWN values below.
+//   3. `device_config.h` is listed in .gitignore — it will not be committed.
+//      Never paste real credentials or tokens into this .example file.
+//   4. Build and flash as normal (idf.py build flash).
+//
+// The pairing token comes from the Quantumancy web app: sign in, create a
+// device from your account (name + optional sensor-type hint), and the
+// backend shows you a raw pairing token EXACTLY ONCE. Copy it straight into
+// DEVICE_PAIRING_TOKEN below — the backend only ever stores a hash of it, so
+// if you lose it your only recourse is deleting the device and pairing a new
+// one.
+
+#pragma once
+
+// --- Wi-Fi station credentials -------------------------------------------
+// The device joins this network in station mode (it does not host an AP).
+#define DEVICE_WIFI_SSID        "your-wifi-ssid-here"
+#define DEVICE_WIFI_PASSWORD    "your-wifi-password-here"
+
+// --- Quantumancy backend ---------------------------------------------------
+// Base URL of the backend, NO trailing slash. The firmware appends
+// "/api/device/telemetry" itself. Use https:// in any real deployment.
+#define DEVICE_BACKEND_BASE_URL "https://your-quantumancy-host.example.com"
+
+// One-time raw pairing token shown by the web app when you create this
+// device (POST /api/device). Sent as `Authorization: Bearer <token>` on
+// every telemetry POST. Treat it like a password.
+#define DEVICE_PAIRING_TOKEN    "paste-your-one-time-pairing-token-here"
+
+// --- Reporting cadence -----------------------------------------------------
+// How often (seconds) the telemetry task samples every registered sensor
+// driver and POSTs a fresh readings batch. The contract caps 1 req/s
+// sustained per device on the backend side — anything >= 2s here is safely
+// inside that budget with room for retries.
+#define DEVICE_REPORT_INTERVAL_SEC 15