cleanup: hoist march_edges' nested cut/march closures to module-level...

[1-Bort-1]

cleanup: hoist march_edges' nested cut/march closures to module-level functions

src/obj_adapter/obj_slice.jl march_edges carries its logic in two nested closures, which CLEAN_CODE.md §6 rules out. Move them to named module-level functions, behaviour unchanged, tests unchanged. Found in #376.

[1-Bort-1]

Hoisting march_edges' nested closures in src/obj_adapter/obj_slice.jl to module-level functions: cut_station, march_stations, and tip_station for the tip bisection block inside march. Behaviour and tests unchanged. Stacked on #376, which rewrites march_edges. Leaving the rest of obj_slice.jl alone.