Fix battery scaling and fan step, add airflow-percentage register

[madsvonqualen]

Fix battery scaling and fan step, add airflow-percentage register

Three related read-decoding fixes, all verified against a live Danfoss Air CCM
(Danfoss Air A2, HW rev. 18.15, SW rev. 2.48).

1. Battery is a raw 0–100 byte, not a 0–255 value

battery_percent (0x01 04 03 0f) was decoded via _read_percent, i.e.
raw * 100 / 255. The register already reports a percentage directly, so a full
battery pack (raw 100) was reported as ≈39 %. Fixed by reading it as a
plain byte.

Confirmed by measuring the Air Dial's 4×AAA pack:

Pack voltage	Raw byte	Old (×100/255)	New (raw)
6.48 V (fresh, 4×1.62 V)	100	39.22	100
5.36 V (depleted)	44	17.25	44

2. fan_step is the stored manual step (1–10), not step×10

The 0.3.0 "fan step in percent" change multiplied the step register
(0x01 04 15 61) by 10 to approximate airflow. But the step is a 1–10 value,
and the real airflow percentage lives in a separate register
(0x00 04 17 81). On the live unit, step 5 coincides with an airflow of
54 % — and 5 × 10 = 50 ≠ 54, so the ×10 was only an approximation.

fan_step now returns the raw manual step, and…

3. New fan_speed_percent register

…the actual airflow percentage is exposed as fan_speed_percent
(0x00 04 17 81, a raw 0–100 byte). Both values are now available and correct,
serving both the default use case and the one from #5.

Live verification

read_all() from the patched client against the CCM:

battery_percent   = 100        (was 39.22)
fan_step          = 5          (was 50)
fan_speed_percent = 54         (new)
humidity          = 61.18      (unchanged: still rescaled)
roomTemperature   = 23.85      (unchanged)
supply_fan_speed  = 1644 / exhaust_fan_speed = 1976
bypass = True / automatic_bypass = True / operation_mode = demand

Tests

Adds tests/test_danfossclient.py — 16 cases covering the changed decoding
(battery raw, fan_step raw, fan_speed_percent) and guarding the unchanged paths
(humidity/filter still rescaled, temperatures in centidegrees, bypass truthiness,
automatic_bypass inversion). All pass on Python 3.14 / pytest 9.

Compatibility note

battery_percent and fan_step now return corrected values, so downstream
consumers that compensated for the old scaling (e.g. multiplying fan_step back
down, or treating battery as 0–39) should drop those workarounds. Would you be
able to cut a release (e.g. 0.4.0) once merged? The Home Assistant integration
will then bump to it.

[JonasPed]

Thanks for the PR. I will however not sit in front of a computer for the next couple of weeks. I will look at it when I am at a computer again.