L2: load_since() for cheap-partial loaders (arbin_res/sql, neware_txt, maccor_txt)

[jepegit]

Epic L of cellpy 2.2 (Stage 5). Design: live-incremental §7 item 2. Depends on L1.

Implement load_since(source, marker) for the sources that can re-read cheaply from a marker first: arbin_res, arbin_sql, neware_txt, maccor_txt. Return native-schema raw rows appended since the marker (may overlap the tail) + the new marker. Other loaders stay full-read and fall back.