docs: Document @micropython.native usage policy.

Closes #405.

Add a guideline to CONTRIBUTING.md: @micropython.native must not be used
in driver code (I2C/SPI bus time dominates, native compilation gains
nothing) but may be used in examples on rendering hot-paths where a
measurable speedup is expected.

Remove the unnecessary @micropython.native from compute_display() in
radar_screen.py — the function does 3 comparisons with no loop, so
native compilation provides no benefit. Also remove the now-unused
`import micropython`.

Author: nedseb

CONTRIBUTING.md

@@ -35,6 +35,7 @@ lib/<component>/
 - **Time**: use `from time import sleep_ms` (not `utime`, not `sleep()` with float seconds).
 - **Exceptions**: use `except Exception:` instead of bare `except:`. Enforced by ruff (E722).
 - **No debug `print()`** in production driver code. Enforced by ruff (T20, examples and tests excluded).
+- **`@micropython.native`**: do **not** use in driver code (`device.py`) — methods are dominated by I2C/SPI bus time, so native compilation gains nothing and reduces debuggability. In **examples**, it may be used on rendering hot-paths (tight pixel loops, math-heavy drawing functions) where a measurable speedup is expected. Do not apply it to functions with only a few comparisons or I/O calls. Note: the native emitter does not support generators, closures, or `with` statements.
 
 ## Driver API conventions
 

lib/vl53l1x/examples/radar_screen.py

@@ -7,7 +7,6 @@
 
 from time import sleep_ms
 
-import micropython
 import ssd1327
 from machine import I2C, SPI, Pin
 from steami_screen import DARK, GRAY, LIGHT, RED, Screen, SSD1327Display
@@ -29,7 +28,6 @@
 tof = VL53L1X(i2c)
 
 
-@micropython.native
 def compute_display(distance):
     """Compute proximity and color from distance value.