…w control

- RecirculationLoop now accepts Pump (LiquidProcessUnit) as inner process, in
  addition to Compressor and ProcessSystem
- DirectMixer/DirectSplitter work with both FluidStream and LiquidStream via
  duck-typed standard_density_gas_phase_after_flash / with_mass_rate()
- LiquidStream.standard_density_gas_phase_after_flash returns density_kg_per_m3
  (standard ≈ actual for incompressible liquids, consistent with legacy PumpModel)
- RecirculationLoop.get_shaft_power_mw() delegates to inner Pump
- All test files updated to use RecirculationLoop/DirectMixer/DirectSplitter directly
  with Sm3/day rates

…bounds to Pump

- ShaftConnectable Protocol: minimum_speed, maximum_speed, set_speed()
- Shaft is now generic Shaft[T: ShaftConnectable] — a shaft drives either
  compressors or pumps, never a mix of both; enforced at the type level
- Shaft._compressors renamed to _units; connect()/get_speed_boundary()
  use the protocol instead of importing Compressor directly
- Compressor gains minimum_speed / maximum_speed properties (delegates to
  compressor_chart) to satisfy ShaftConnectable
- Pump gains minimum_speed / maximum_speed properties (delegates to pump_chart)
  to satisfy ShaftConnectable — a pump shaft is now a VariableSpeedShaft[Pump]

…otocol

- Add RecirculatingUnit protocol (get_id, maximum_flow_rate, get_recirculation_range)
- Pump implements RecirculatingUnit via minimum_flow_rate, maximum_flow_rate, get_recirculation_range()
- Replace Compressor with RecirculatingUnit in all strategy classes:
  - anti_surge/common_asv.py: first_compressor → first_unit
  - anti_surge/individual_asv.py: compressors → units
  - pressure_control/common_asv.py: first_compressor → first_unit
  - pressure_control/individual_asv.py: compressors → units
- Replace FluidStream with Any in all solver/strategy method signatures:
  - outlet_pressure_solver.py, anti_surge_strategy.py, pressure_control_strategy.py
  - common_asv.py (both anti_surge and pressure_control), individual_asv.py (both)
- ProcessRunner.run() and ProcessSystemRunner._propagate_stream_to_id() accept Any stream
- Update all call sites in mapper and tests (first_compressor= → first_unit=, compressors= → units=)
- Fix pump._head_and_efficiency → _head_and_efficiency_at_rate rename

…t to GasProcessUnit

- ProcessUnit is now the stream-agnostic protocol (get_id + propagate_stream)
- GasProcessUnit is the gas-specific ABC extending Entity
- Detach DirectMixer/DirectSplitter from ABC hierarchy (standalone, uuid7)
- Move protocol from stream_protocol.py to process_pipeline/process_unit.py
- Add pump integration tests (speed solver, shaft power, recirculation)
- Clean up FluidStream.standard_density docstring

- Rename first_compressor → first_unit in new ASV test factories
- Fix FluidStream → StreamWithPressure type annotation in individual_asv.py
- xfail pump recirculation test pending boundary calculation investigation

The regression test from PR #1515 used first_compressor= kwarg which was
renamed to first_unit= on this branch. Also remove xfail from pump
recirculation test since the underlying bug is now fixed on main.

Widen choke solver and pressure control strategy signatures from
FluidStream to StreamWithPressure for consistency. Add pyright ignore
comments where cached_property on FluidStream prevents structural
protocol matching (a known basedpyright limitation with cached_property
vs @Property in protocols). Add propagate_stream to RecirculatingUnit
protocol.

Replace Generic[_T] with PEP 695 class Shaft[T: ShaftConnectable] syntax,
as required by ruff UP046.