fix: accumulate serial chunks in _read_metadata to prevent calibration data loss

[valentinkauf]

Problem

_read_metadata() reads the PPK2 metadata string in a loop but returns
as soon as it finds a single serial buffer chunk containing "END",
discarding all previously received chunks.

The PPK2 firmware sends its full calibration metadata as a multi-line
ASCII string ending with END. Depending on the OS USB serial driver
and buffer sizes, this string arrives split across multiple read buffers.
When that happens, calibration fields that arrived in earlier chunks —
O, S, I, GS, GI, HW, etc. — are silently discarded and
remain at their default values of 0.

Concrete impact:
At the lowest measurement range (range 0, ~µA scale), O[0] is the
factory-measured ADC zero-point offset (typically ~166 ADC counts). With
O[0] = 0 (default), the calibration formula:

result = (adc - O[0]) × (adc_mult / R[0])

applies no zero-point correction, adding a fixed ~1.8 µA offset to every
sample. This makes the library report systematically wrong values for any
target consuming in the low-µA range, with no warning or error.

The nRF Connect Power Profiler application (the official Nordic reference)
accumulates all chunks: metadata += chunk until "END" appears. It
never loses calibration data and always reads correctly.

Reproduction

Run on a factory-calibrated PPK2 with a low-current (< 5 µA) load.
Compare the average reading from this library to nRF Connect — the
library reads ~1.8 µA higher.

Enabling verbose logging of the raw metadata string reveals the issue:
fields like HW: and O0:–O4: are absent from what the library
received, even though the device sent them.

Fix

Accumulate all serial chunks before checking for "END", matching nRF
Connect behaviour. The number of loop iterations is increased from 5 to
10 to accommodate devices that may require more reads.

Tested on

• macOS, PPK2 firmware with factory calibration (Calibrated: 0)
• Metadata spans 2 serial read buffers on this platform
• After fix: library reading matches nRF Connect within ±0.1 µA