Add bh1750, aht20, sgp40 drivers unified on native i2c_master API

Forked from managed_components (espressif/bh1750, espressif/aht20,
esp-idf-lib/sgp40) and rewrote aht20/sgp40 internals to use
driver/i2c_master.h directly instead of the i2c_bus and i2cdev
wrapper components, so all three sensors share one create/delete
handle pattern over a single i2c_master_bus_handle_t.

README.md

@@ -0,0 +1,47 @@
+# espidf_i2c
+
+Forked/rewritten ESP-IDF I2C sensor drivers, unified onto the native
+`driver/i2c_master.h` API (ESP-IDF >= 5.3) so every sensor shares the same
+handle lifecycle instead of each vendor bringing its own I2C abstraction
+(`i2c_bus`, `i2cdev`, etc.).
+
+## Common API convention
+
+Every driver in this repo follows the same shape:
+
+```c
+typedef void *xxx_handle_t;
+
+esp_err_t xxx_create(i2c_master_bus_handle_t bus_handle, uint8_t dev_addr, xxx_handle_t *handle_ret);
+esp_err_t xxx_delete(xxx_handle_t handle);
+// ... device-specific read/measure functions taking xxx_handle_t
+```
+
+Callers create one `i2c_master_bus_handle_t` via `i2c_new_master_bus()` and
+pass it to each sensor's `_create()`, which internally calls
+`i2c_master_bus_add_device()`. No sensor wraps the bus a second time.
+
+## Components
+
+- `bh1750/` — ambient light sensor. Already used the native API upstream;
+  relocated here unchanged.
+- `aht20/` — temperature/humidity sensor. Rewritten from Espressif's
+  `i2c_bus` component onto `i2c_master_transmit`/`i2c_master_receive`.
+- `sgp40/` — VOC index sensor. Rewritten from esp-idf-lib's `i2cdev`
+  component onto `i2c_master_transmit`/`i2c_master_receive`; the Sensirion
+  VOC algorithm files are carried over unmodified (no I2C dependency).
+
+## Using from a project
+
+```yaml
+dependencies:
+  laz/bh1750:
+    git: https://git.jeese.fr/laz/espidf_i2c.git
+    path: bh1750
+  laz/aht20:
+    git: https://git.jeese.fr/laz/espidf_i2c.git
+    path: aht20
+  laz/sgp40:
+    git: https://git.jeese.fr/laz/espidf_i2c.git
+    path: sgp40
+```