v1.3.4

feat: add fronius.h and registers.h umbrella headers

Consumers currently must include several individual headers to use
the library, e.g.:

  #include <fronius/fronius_bus.h>
  #include <fronius/fronius_device.h>
  #include <fronius/inverter.h>
  #include <fronius/meter.h>
  #include <fronius/modbus_config.h>
  #include <fronius/modbus_error.h>

Add two umbrella headers reflecting the two natural tiers of the
public API:

- fronius.h: the core object API (bus, device, inverter, meter,
  config, error, types) needed by consumers driving devices through
  the high-level Inverter/Meter interface.

- registers.h: raw Modbus register access (SunSpec common block,
  per-device register maps, register_base, and the ModbusUtils
  pack/unpack helpers) needed by consumers that build a virtual
  Modbus TCP slave mapping or otherwise touch raw registers directly.

Both are header-only and require no CMake changes, since headers
are already installed flatly under include/fronius/ via the existing
install(DIRECTORY include/ ...) rule.

Verified with g++ -std=c++23 and a warning-free Doxygen run.