docs: describe battery cell struct registers and bat_mng_struct profile layouts

[pluskal]

This PR adds documentation only — no functional changes: description= strings for the struct-type battery registers and the bat_mng_struct.profile_* object IDs in registry.py (they flow into the generated registry CSV tables), plus a short "Struct-type registers" section on the registry docs page summarizing the payload layouts.

Verification

All layouts were verified live today against a Power Storage 6.0 (BMS software 5615, 4 battery modules) via the port 8899 protocol:

• battery.cells[N] — 96 bytes = 24 records of 4 bytes: temperature [°C] uint8, cell voltage [mV] uint16 LE, status uint8. Cross-checked against battery.min_cell_voltage / battery.max_cell_voltage (matches to 0.1 mV) and battery.temperature (~30 °C, matches).
• battery.cells_resist[N] — 96 bytes = 24 records of resistance uint16 BE (unit 1/256 mOhm) + 2 bytes padding. Matches the decoder already merged in Add support for cell resistance data #51; format originally determined by @sfudeus (branch cell_resist).
• battery.cells_stat[N] — 48 bytes = u_min / u_max / t_min / t_max, each (cell index uint32 LE (0-based), unix timestamp uint32 LE, value float32 LE). Matches the decoder merged in Add explicit support for battery cell data and battery module history #48; format by @sfudeus (branch battery_data_decoding). Verified that the decoded values match the granular battery.cells_stat[N].u_max.value/.index/.time registers.
• bat_mng_struct.profile_pdc / profile_pdc_max / profile_load / profile_pext — 192 bytes = <48f little-endian, one Watt value per 30-minute slot of the day starting at 00:00, as first described by @SaxonWood in Decoding bat_mng_struct.profile... OIDs #49. Verified to decode into plausible daily curves (PV ~31 kWh, load ~34 kWh for the day). Per the ticket, no decoder is added here — descriptions only.

The descriptions on the granular battery.cells_stat[N].* subfields were left untouched.

src/rctclient/registry.py still parses (ast.parse) and the description column alignment follows the existing file style.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR enhances the registry/documentation for “struct-type” inverter registers by adding description strings for composite battery-related OIDs and by documenting the binary payload layouts in the registry docs page, to make reverse-engineered formats discoverable via the generated CSV tables and Sphinx docs.

Changes:

• Added description= strings for battery.cells[N], battery.cells_resist[N], and battery.cells_stat[N] struct-type OIDs in registry.py.
• Added description= strings for bat_mng_struct.profile_* OIDs describing their 48×float32 (192-byte) daily profile payload layout.
• Added a “Struct-type registers” section to docs/inverter_registry.rst summarizing the verified binary layouts.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/rctclient/registry.py	Adds descriptions for composite/struct-type battery OIDs and bat_mng_struct.profile_* layout notes (used in generated registry CSV).
docs/inverter_registry.rst	Documents the struct-type register payload layouts and references the existing decoding types where applicable.

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[pluskal]

Good catch — fixed in a5d5ce2. All three are now DataType.UNKNOWN, matching profile_load which already used it; the CLI then falls back to the hexdump path in cli.py instead of attempting an ASCII decode of float32 payloads. A dedicated 48×float32 decoder type would be a natural follow-up but is out of scope for this docs PR.