Add support for SPH in V1 API

[johanzander]

Overview

Adds complete V1 API support for SPH (type 5) hybrid inverters

Design Rationale (per #125 discussion)

• Zero breaking changes - all existing min_* methods unchanged, backward compatible
• Follows the [perfect PR recommendations][perfect-pr]
• Follows existing architectural patterns - sph_* methods mirror min_* structure exactly
• Potential improvements can be done in subsequent PRs

What's Included

10 new SPH methods:

  api.sph_detail(device_sn)                    # Device details
  api.sph_energy(device_sn)                    # Current energy
  api.sph_energy_history(...)                  # History (7-day max)
  api.sph_settings(device_sn)                  # All settings
  api.sph_read_parameter(...)                  # Read parameter
  api.sph_write_parameter(...)                 # Write parameter
  api.sph_write_ac_charge_time(...)            # AC charge periods (1-3)
  api.sph_write_ac_discharge_time(...)         # AC discharge periods (1-3)
  api.sph_read_ac_charge_times(...)            # Read charge periods
  api.sph_read_ac_discharge_times(...)         # Read discharge periods

  DeviceType enum:
  from growattServer import DeviceType  # SPH=5, MIN=7, etc.

Documentation:

• README updates with all method signatures
• Example script (examples/sph_example.py)

Technical Details

• Endpoints: device/mix/*, mixSet, readMixParam (SPH uses MIX endpoints)
• Time Periods: 3 AC charge + 3 AC discharge periods per API specification
• Review: Parameter ranges match official Growatt V1 API docs

Testing

• All methods verified against https://www.showdoc.com.cn/262556420217021
• Parameter structures match API spec tables
• Zero impact on existing MIN implementation
• NO verification agains real hardware

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Related: Based on #125 discussion; independent implementation from master branch.
[perfect-pr]: https://developers.home-assistant.io/docs/review-process/#creating-the-perfect-pr