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/main/CMakeLists.txt

@@ -0,0 +1,4 @@
+idf_component_register(
+    SRCS "test_sensor.cpp"
+    PRIV_REQUIRES sensors unity
+)

components/sensors/test_apps/host/main/test_sensor.cpp

@@ -0,0 +1,85 @@
+#include "sensor.h"
+#include "unity.h"
+
+using namespace kvida;
+
+namespace {
+
+// Minimal fake sensor exercising the fetch()/get() cache contract and
+// SensorValue's fixed-point convention (val1 + val2 * 1e-6).
+class FakeSensor : public Sensor {
+public:
+    const char *id() const override { return "fake0"; }
+    void begin() override {}
+
+    bool supports(SensorChannel channel) const override
+    {
+        return channel == SensorChannel::AmbientTemperature;
+    }
+
+    bool fetch() override
+    {
+        fetched_ = true;
+        return true;
+    }
+
+    bool get(SensorChannel channel, SensorValue &out) override
+    {
+        if (!fetched_ || channel != SensorChannel::AmbientTemperature) {
+            return false;
+        }
+        out = SensorValue{21, 500000, "C", 1234};
+        return true;
+    }
+
+private:
+    bool fetched_ = false;
+};
+
+double to_double(const SensorValue &v)
+{
+    return v.val1 + v.val2 * 1e-6;
+}
+
+} // namespace
+
+TEST_CASE("get fails before fetch", "[sensor]")
+{
+    FakeSensor sensor;
+    SensorValue value{};
+    TEST_ASSERT_FALSE(sensor.get(SensorChannel::AmbientTemperature, value));
+}
+
+TEST_CASE("fetch then get returns the cached value", "[sensor]")
+{
+    FakeSensor sensor;
+    sensor.begin();
+    TEST_ASSERT_TRUE(sensor.fetch());
+
+    SensorValue value{};
+    TEST_ASSERT_TRUE(sensor.get(SensorChannel::AmbientTemperature, value));
+    TEST_ASSERT_EQUAL_DOUBLE(21.5, to_double(value));
+}
+
+TEST_CASE("get fails for an unsupported channel", "[sensor]")
+{
+    FakeSensor sensor;
+    sensor.begin();
+    sensor.fetch();
+
+    SensorValue value{};
+    TEST_ASSERT_FALSE(sensor.get(SensorChannel::Humidity, value));
+}
+
+TEST_CASE("default set_trigger reports unsupported", "[sensor]")
+{
+    FakeSensor sensor;
+    TEST_ASSERT_FALSE(sensor.set_trigger(SensorTriggerType::DataReady, nullptr, nullptr));
+}
+
+extern "C" void app_main(void)
+{
+    UNITY_BEGIN();
+    unity_run_all_tests();
+    UNITY_END();
+}