Add sensors Sensor interface and a verified host test setup

Sensor/SensorValue in components/sensors/include/sensor.h is modeled
on Zephyr's sensor API: a fetch()/get() split so multi-channel sensors
(e.g. BME280) pay the hardware read cost once, fixed-point SensorValue
(no float/double -- ESP32-C6 has no hardware FPU), a fine-grained
SensorChannel enum, and an optional interrupt-driven set_trigger() for
the battery-first sleep/wake design.

Adds components/sensors/test_apps/host, a self-contained ESP-IDF
project building against the `linux` target so this logic is
host-testable per AGENTS.md's testing philosophy -- verified in a
clean run against the pinned espressif/idf:v6.0.2 image (builds, 4/4
tests pass). Replaces the earlier generic top-level test/README.md
placeholder now that a real per-component pattern exists.

Also fixes .gitignore's build/managed_components patterns to match
nested paths, not just the repo root.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

components/sensors/test_apps/host/CMakeLists.txt

@@ -0,0 +1,18 @@
+# Host-based unit tests for the `sensors` component, run on ESP-IDF's
+# `linux` target (no board needed). Build/run with:
+#
+#   idf.py --preview set-target linux
+#   idf.py build
+#   ./build/test_sensor_host.elf
+#
+# COMPONENTS is restricted (mirrors the pattern used by ESP-IDF's own
+# in-tree host test apps, e.g. components/cxx/test_apps/*) so the build
+# doesn't try to pull in board-only components that don't build for
+# linux.
+cmake_minimum_required(VERSION 3.16)
+
+set(EXTRA_COMPONENT_DIRS "${CMAKE_CURRENT_LIST_DIR}/../../../")
+set(COMPONENTS main sensors unity)
+
+include($ENV{IDF_PATH}/tools/cmake/project.cmake)
+project(test_sensor_host)