Support adjust_node_capacity in C++ mode and define threads on World for both modes

[toruseoagent]

Summary

Two follow-up changes for the C++ mode:

1. adjust_node_capacity support in C++ mode — ports the feature added for the Python mode in add adjust_node_capacity option #344 to the C++ engine, so World(cpp=True, adjust_node_capacity=True) behaves the same as the Python mode.
2. threads defined on World for both modes — the threads argument introduced in Parallelize the C++ engine with deterministic OpenMP while keeping the AoS data layout #341 was a cpp-mode-only extension of the wrapper, so World(cpp=False, threads=N) raised TypeError. It is now a regular World.__init__ parameter documented in uxsim.py; the Python mode accepts and simply ignores it.

Details

adjust_node_capacity (commit 1)

• Node::adjust_node_capacity() in the C++ engine mirrors the Python implementation line by line: if flow_capacity is unset (C++ sentinel -1.0 corresponding to Python None) and the node has both in- and out-links, it sets flow_capacity = (max(inlink capacities) + max(outlink capacities)) / 2, updates flow_capacity_remain = flow_capacity * DELTAT, and derives number_of_lanes = ceil(flow_capacity / 0.8) when lanes are unset (flagging flag_lanes_automatically_determined).
• Applied to all nodes at scenario finalization (initialize_adj_matrix), the same timing as the Python mode's finalize_scenario. Idempotent once flow_capacity >= 0.
• The wrapper exposes CppNode.adjust_node_capacity() and syncs the wrapper-side node attributes after finalization.
• Validation: on a 6x6 grid mixing 2-lane major and 1-lane minor roads (8000 vehicles, 5 seeds), the resulting flow_capacity and number_of_lanes of every node are identical between Python and C++ modes; TTT differs by +0.89% on average (expected RNG-sequence effect).

threads on World (commit 2)

• threads: int=1 is inserted in World.__init__ immediately before cpp, matching the positional layout of the wrapper's CppWorld.__init__, with a numpydoc entry (default 1; N≥1 threads; -1 = all cores; ignored when cpp=False).
• The Python mode performs no validation and stores nothing — the argument simply has no effect there. The C++ mode keeps the existing validation (bool rejected, ValueError for 0 / ≤ -2) and forwarding to the engine.

Tests

• tests/test_cpp_mode.py: 217 passed — three adjust_node_capacity tests ported from tests/test_verification_node.py (merge capacity, major/minor free-flow, major/minor congested), and test_threads_parameter extended with a Python-mode block asserting World(cpp=False) with threads=4 / threads=-1 runs and produces results identical to omitting the argument.
• tests/test_other_functions.py: 28 passed (1 pre-existing environment-only failure: osmnx not installed).

Part of #297

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