Fix struct.error crashes on Conext SW 4024 / MPPT 60 150 (shorter fast-packet payloads)

[kaanow]

The problem

Running berrybms against a Conext SW 4024 + MPPT 60 150 site crashes in every fast-packet processor: this device generation sends shorter payloads than the XW+ / MPPT 80 600 the code was developed on, and the fixed-size struct.unpack calls raise struct.error.

Observed payload lengths (from a live capture corpus, ~59k reassembled messages):

PGN	XW+ / MPPT 80 600 (this repo)	SW 4024 / MPPT 60 150
BattSts2 (0x1F0C4)	41	36
DcSrcSts2 (0x1F0C5)	27	21
ChgSts (0x1F00E)	20	17
AcStsRms (0x1F016)	55 / 83	78

The fix

The field layouts are prefix-stable — the older devices just send fewer trailing bytes. (Verified: battery voltage decodes at your documented offsets on the SW 4024 and matches the same site's InsightHome Modbus registers at r = +0.98.) So:

• switch each processor to struct.unpack_from + a minimum-length guard;
• branch processAcStsRms on the association id (0x13 gen / 0x33 loads / 0x43 grid — per your own comments) instead of the exact payload length, and only read the extra grid block past byte 55 when it's present.

Testing

• 59k real reassembled payloads from the SW 4024 / MPPT 60 150 bus through all four processors: no exceptions, values sensible (battery/PV V/I/W, charge stage all cross-checked against Modbus).
• Regression: the full-length sample payloads from this file's own comments (27 B DcSrcSts2 → 266.56 V / 1356 W, 41 B BattSts2) decode unchanged.

Two related findings from the same site that may interest you, documented in our fork's findings notes:

1. The MPPT 60 150 never populates PV-side current/power in DcSrcSts2 assoc 0x15 (hard zeros even at full production, and the InsightHome's Modbus PV-current registers are zero too) — production on that model is only available via the assoc 0x03 output channel.
2. The assoc values you identified empirically match the official Xantrex XB_eAC_SRC_ID/XB_eDC_SRC_ID enums leaked in the Freedom SW-RVC DGN Reference Guide (976-0452-01-01): 0x03=HOUSE_BAT_BANK1, 0x13=GEN1, 0x15=SOLAR_ARRAY1, 0x33=AC_LOAD1, 0x43=GRID1.

(Independent of #3, which fixes >16-frame reassembly; the two can merge in either order.)