Improve robustness of the Arad / Master Meter Dialog3G decoder.

Preamble detection now uses a configurable nibble window inside the
observed 48-bit preamble while payload extraction always aligns to the
start of the full preamble. This keeps decoding stable even if the match
window changes.

Add a diagnostic field "unmatched_preamble" which outputs the preamble
nibbles outside the match window in inverted nibble form:

    <before>_..._<after>

This helps analyze variants of the Dialog3G transmission.

Add decoder options:
- mandatory serial input
- gear override
- unit override

These allow forcing decoding parameters when meter configuration or
protocol variants differ from the automatically detected values.

Update protocol documentation to reflect observed messages:

* the trailing FF byte is not constant
* the serial suffix byte sometimes matches the printed meter letter
* the suffix byte together with the following byte likely indicates
  meter unit and gear ratio

Add BitBench alignment reference:

    PREAMBLE_ALIGN = c196f51385

(invert of 3e690aec7a)

Format string used for analysis:

    UID:16h SERIAL:<24d 8h 8h
    COUNTER:<32d 8h8h 8h8h 8h8h
    SUFFIX:hh