Remove dead code from C++ engine and bindings (-254 lines)

[toruseoagent]

Summary

Remove 254 lines of C++ functions and Python bindings that are never called through the public API or internal wrapper paths.

Removed functions

traffi.cpp (-127 lines):

Function	Why dead
Vehicle::log_state_str()	Wrapper converts states in Python
Vehicle::log_link_names()	Wrapper converts link IDs in Python
Vehicle::build_log_t_link()	Wrapper uses build_full_log_np instead
World::get_vehicle(name)	Wrapper uses get_vehicle_by_index
World::get_link_by_id(id)	Wrapper uses get_link(name)
World::get_vehicles_by_state()	Never called from wrapper
World::get_vehicle_states_by_index()	Never called from wrapper

bindings.cpp (-107 lines):

Binding	Why dead
build_full_log (string version)	build_full_log_np is used instead
get_log_state_strings	Wrapper does conversion in Python
get_log_t_link_data	Wrapper does conversion in Python
Node generation_queue property	Not exposed through wrapper
Link vehicles list property	Wrapper uses its own deque

traffi.h (-20 lines): Corresponding declarations.

Verification method

Each function was confirmed dead by:

1. grep across entire codebase (*.py, *.cpp, *.h) for all callers
2. Tracing the wrapper's call chains to confirm no indirect usage
3. All 185 existing C++ mode tests pass without modification

Test plan

• All 185 C++ mode tests pass (--reruns 5)
• All 8 test_cpp_build.py tests pass
• No test changes required

Part of #297

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR removes C++ engine helper methods and nanobind bindings that are not used by UXsim’s Python wrapper paths, reducing the C++ surface area and maintenance burden in the cpp=True backend.

Changes:

• Removed unused Vehicle log string/label helpers and World lookup/filter helpers from the C++ engine.
• Removed unused nanobind-exposed methods/properties (e.g., legacy per-vehicle string log builders, Node.generation_queue, Link.vehicles, and several World query helpers).
• Kept the wrapper-preferred fast-path APIs (notably build_full_log_np, batched log building, and index-based lookups).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
uxsim/trafficpp/traffi.h	Removes declarations for unused helper methods so headers match the trimmed implementation/bindings.
uxsim/trafficpp/traffi.cpp	Deletes implementations of dead helper functions (string conversions, name/id lookup helpers, state filtering helpers).
uxsim/trafficpp/bindings.cpp	Drops nanobind exports for the removed helpers/properties to keep the Python extension API aligned with the wrapper’s actual usage.