Cap achieved-speed re-solves to fix native stack overflow in set_ach_speed

[jhoshiko]

set_ach_speed recursed without bound on some real-world trips, killing the process with a stack overflow that Python could not catch. This patch adds a maximum-depth sim param and a guard that returns a catchable error instead.

• Adds SimParams.ach_speed_max_depth (default 100)
• Adds a depth counter through a private helper, SimDrive::set_ach_speed_at_depth, which the public set_ach_speed now wraps to avoid changes to the function signature
• The default value of 100 was chosen after a sweep across 312k real trips over 40 vehicles, with the deepest terminating trip needing 3 nested solves, and on the 337 crash cycles from the original report the deepest that still terminates needed 50. Nothing terminates between 50 and 100, and the crash cycles hit the cap at 100 and again at a probe cap of 500, so they are genuinely unbounded.
• Adds a regression test with a synthetic infeasible cycle and a cap set to 1 to test the new depth guard

[jhoshiko]

Closes #326