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>

sdkconfig.defaults

@@ -4,3 +4,7 @@
 
 CONFIG_PARTITION_TABLE_CUSTOM=y
 CONFIG_PARTITION_TABLE_CUSTOM_FILENAME="partitions.csv"
+
+# Matches the 4MB layout assumed in partitions.csv (TODO: adjust both
+# together once real hardware's actual flash size is confirmed).
+CONFIG_ESPTOOLPY_FLASHSIZE_4MB=y