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/app_main.c

@@ -0,0 +1,51 @@
+// Quantumancy ESP32-P4 Sensor Node — entry point.
+//
+// UNVERIFIED AGAINST REAL HARDWARE. Nobody working on this workstream has a
+// physical ESP32-P4 (or the BME280 / LD2410 modules) to flash and test
+// against. This firmware is real, structurally-sound ESP-IDF C, reasoned
+// about carefully against ESP-IDF's documented APIs and the public
+// datasheets/protocol docs for each sensor -- but "compiles and reads
+// correctly" is where the verification stops. See README.md's "What's
+// verified vs. not" section before treating any of this as field-tested.
+//
+// Boot sequence: bring up Wi-Fi station mode (device_config.h credentials),
+// wait (briefly, non-fatally) for an initial connection, initialize every
+// registered sensor driver, then hand off to the telemetry task, which
+// periodically samples the sensor registry and POSTs the results to the
+// backend. Wi-Fi reconnection and per-cycle "are we online" checks happen
+// independently after this, so a boot-time Wi-Fi hiccup doesn't wedge the
+// device -- it just starts reporting once the connection comes up.
+
+#include "wifi_manager.h"
+#include "sensor_registry.h"
+#include "telemetry_client.h"
+#include "esp_err.h"
+#include "esp_log.h"
+#include "freertos/FreeRTOS.h"
+#include "freertos/task.h"
+
+static const char *TAG = "app_main";
+
+// How long to wait at boot for the first Wi-Fi connection before giving up
+// on blocking and handing off to the telemetry task anyway (which will
+// simply skip cycles until wifi_manager's own retry logic connects).
+#define BOOT_WIFI_WAIT_MS 20000
+
+void app_main(void) {
+    ESP_LOGI(TAG, "Quantumancy sensor node starting");
+
+    ESP_ERROR_CHECK(wifi_manager_start());
+
+    esp_err_t err = wifi_manager_wait_connected(pdMS_TO_TICKS(BOOT_WIFI_WAIT_MS));
+    if (err == ESP_OK) {
+        ESP_LOGI(TAG, "Wi-Fi connected at boot");
+    } else {
+        ESP_LOGW(TAG, "Wi-Fi not connected within %d ms at boot -- continuing anyway, "
+                       "wifi_manager will keep retrying in the background", BOOT_WIFI_WAIT_MS);
+    }
+
+    sensor_registry_init_all();
+    telemetry_client_start_task();
+
+    ESP_LOGI(TAG, "startup complete, telemetry task running");
+}