Map nRF905 payload bytes to physical readings

[andy778]

Why

Once we can decode frames, we still need to know what each payload byte means (water level? temperature? pump state? alarm flags?). That mapping can only come from watching the real unit.

What to do (after #52 has captures)

1. Decode captured frames to payload hex (rtl_433 flex decoder or the stream/rtl433 decoder once address/CRC are filled in).
2. For each capture, write down what the unit's display/LEDs/state showed at that moment.
3. Diff payloads across states: bytes that change with water level vs. that toggle on pump/alarm reveal the field layout.
4. Record the byte→field mapping so the decoder's data_make() fields can be filled in (replacing the current placeholders).

Deliverable

• A byte-offset → meaning table (even partial), added to notes/rtl433.md.

Blocked by: #52 (need captures first).