lis2mdl: Auto-trigger single conversion in idle mode.

[nedseb]

Closes #70

Summary

When the LIS2MDL is in idle mode (power_down() called), reading magnetic field or temperature data now automatically triggers a single conversion and waits for data ready, instead of returning stale values.

• _ensure_data() — checks is_idle(), sets mode to "single", polls data_ready() (up to 100ms)
• Called in read_magnet() and read_temperature_raw()
• All higher-level methods (read_magnet_uT(), magnitude_uT(), read_temperature_c(), heading_flat_only(), etc.) benefit automatically

Follows the same convention as WSEN-PADS, WSEN-HIDS, HTS221, and ISM330DL (#44).

Test plan

Mock tests (no hardware)

python3 -m pytest tests/ -k "lis2mdl and mock" -v

tests/test_scenarios.py::test_scenario[lis2mdl/Verify WHO_AM_I register/mock] PASSED
tests/test_scenarios.py::test_scenario[lis2mdl/Read WHO_AM_I via method/mock] PASSED
tests/test_scenarios.py::test_scenario[lis2mdl/Read status register/mock] PASSED
tests/test_scenarios.py::test_scenario[lis2mdl/Read magnetic field returns tuple/mock] PASSED
tests/test_scenarios.py::test_scenario[lis2mdl/Read magnetic field in uT returns tuple/mock] PASSED
tests/test_scenarios.py::test_scenario[lis2mdl/Read temperature returns float/mock] PASSED
tests/test_scenarios.py::test_scenario[lis2mdl/Temperature with offset/mock] PASSED
tests/test_scenarios.py::test_scenario[lis2mdl/Temperature with two-point calibration/mock] PASSED
tests/test_scenarios.py::test_scenario[lis2mdl/Magnetic field readable after power down/mock] PASSED
tests/test_scenarios.py::test_scenario[lis2mdl/Temperature readable after power down/mock] PASSED
tests/test_scenarios.py::test_scenario[lis2mdl/No auto-trigger when already active/mock] PASSED

11 passed

Hardware tests (STeaMi board connected)

python3 -m pytest tests/ --port /dev/ttyACM0 -k "lis2mdl and hardware" -s -v

tests/test_scenarios.py::test_scenario[lis2mdl/Verify WHO_AM_I register/hardware] PASSED
tests/test_scenarios.py::test_scenario[lis2mdl/Read WHO_AM_I via method/hardware] PASSED
tests/test_scenarios.py::test_scenario[lis2mdl/Read status register/hardware] PASSED
tests/test_scenarios.py::test_scenario[lis2mdl/Fresh magnitude after power down/hardware] PASSED
tests/test_scenarios.py::test_scenario[lis2mdl/Fresh temperature after power down/hardware] PASSED
tests/test_scenarios.py::test_scenario[lis2mdl/Magnitude in plausible range/hardware] PASSED
tests/test_scenarios.py::test_scenario[lis2mdl/Temperature in plausible range/hardware] PASSED

7 passed, 4 skipped

[nedseb]

Observed behavior confirming the need for _ensure_data()

Running the LIS2MDL hardware tests in sequence shows a magnitude inconsistency caused by stale data after soft_reset() / reboot():

tests/test_scenarios.py::test_scenario[lis2mdl/Magnitude in plausible range/hardware] 178.19 PASSED
tests/test_scenarios.py::test_scenario[lis2mdl/Temperature in plausible range/hardware] 5.75 PASSED
tests/test_scenarios.py::test_scenario[lis2mdl/Soft reset then WHO_AM_I/hardware] 0x40 PASSED
tests/test_scenarios.py::test_scenario[lis2mdl/Reboot then WHO_AM_I/hardware] 0x40 PASSED
tests/test_scenarios.py::test_scenario[lis2mdl/Magnetic field values feel correct/hardware]
  Magnitude: 0.21 µT    <-- stale/zero data after reboot put sensor in idle
  Temperature: 5.75 °C
  Heading: 236.45 °

• First magnitude read (178.19 µT): sensor was in continuous mode from init, but the value is abnormally high (~25-65 µT expected). May indicate that the first read arrived before stable data was available.
• Second magnitude read (0.21 µT): after soft_reset() and reboot() tests, the sensor is back in idle mode. magnitude_uT() reads stale/zero registers without triggering a new conversion.

This confirms the _ensure_data() pattern is needed: read_magnet() and read_temperature_raw() should detect idle mode and trigger a single conversion before reading.

Additional observation

The temperature reads 5.75°C at ~22°C ambient — this is a separate issue tracked in #97 (missing offset correction in read_temperature_c()).

[Copilot]

Pull request overview

This PR updates the LIS2MDL driver so that read methods automatically trigger a one-shot conversion when the device is in idle mode (after power_down()), aligning behavior with the repository’s “auto-trigger in power-down/idle” convention.

Changes:

• Add LIS2MDL._ensure_data() to switch to single-conversion mode and wait for data_ready() when the sensor is idle.
• Call _ensure_data() from read_magnet() and read_temperature_raw() so higher-level APIs inherit the behavior.
• Add new scenario tests for “read after power down” behavior in both mock and hardware modes.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
lib/lis2mdl/lis2mdl/device.py	Adds _ensure_data() and hooks it into magnet/temperature reads to auto-trigger conversions in idle mode.
tests/scenarios/lis2mdl.yaml	Adds mock + hardware scenarios intended to validate “fresh reads after power down” behavior.

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