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/sensor_registry.h

@@ -0,0 +1,31 @@
+// Sensor driver registry: the one place that knows the concrete list of
+// sensor drivers compiled into this firmware. app_main.c and
+// telemetry_client.c never reference bme280.c / ld2410.c directly -- they
+// only talk to this registry, so adding a new sensor never touches them.
+//
+// See README.md's "Adding a new sensor" section for the step-by-step.
+
+#pragma once
+
+#include "sensor_driver.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+// Calls every registered driver's init() (in array order), logging and
+// continuing past individual failures rather than aborting -- one dead
+// sensor shouldn't take the whole node offline. Call once at boot, before
+// the telemetry task starts.
+void sensor_registry_init_all(void);
+
+// Calls every registered driver's read() and appends its readings into
+// `out` (capacity `max_out`), writing the total count written to
+// `*out_count`. A driver that errors is logged and simply contributes zero
+// readings this cycle. Returns ESP_OK; this function does not fail as a
+// whole just because one driver did.
+esp_err_t sensor_registry_collect(sensor_reading_t *out, size_t max_out, size_t *out_count);
+
+#ifdef __cplusplus
+}
+#endif