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/sdkconfig.defaults

@@ -0,0 +1,25 @@
+# Default sdkconfig overrides for the Quantumancy sensor node.
+#
+# These are seed values applied when `idf.py set-target esp32p4` generates the
+# real `sdkconfig` (which is machine/toolchain-specific and NOT committed —
+# see .gitignore). UNVERIFIED: nobody has run `idf.py build` against a real
+# ESP-IDF install for this project, so treat these as reasonable, standard
+# starting points rather than confirmed-working values.
+
+CONFIG_IDF_TARGET="esp32p4"
+
+# The backend is served over HTTPS in production (see deploy/), so the HTTP
+# client needs mbedTLS's bundled CA store to verify the TLS cert.
+CONFIG_MBEDTLS_CERTIFICATE_BUNDLE=y
+CONFIG_MBEDTLS_CERTIFICATE_BUNDLE_DEFAULT_FULL=y
+
+# Headroom for the main task: JSON body construction (cJSON) plus
+# esp_http_client happen on it via the telemetry task.
+CONFIG_ESP_MAIN_TASK_STACK_SIZE=4096
+
+# A handful of sockets is plenty for one outbound HTTP client + Wi-Fi/LWIP
+# housekeeping; keeps RAM use predictable on a sensor node.
+CONFIG_LWIP_MAX_SOCKETS=16
+
+# Reasonably quiet serial log output by default (bump to DEBUG for bring-up).
+CONFIG_LOG_DEFAULT_LEVEL_INFO=y