Refactor the time iteration loop

Problem

The main time-stepping loop lives in the free function iterate_solution
(Code/Source/solver/main.cpp#L159). It hard-codes a single solution strategy: each time step runs the nonlinear (Newton) solve once. There is no extension point for schemes that need a different per-step structure, e.g.:

• Load stepping for G&R problems (apply the load in increments, with a
  nonlinear solve per increment).
• Alternative coupling drivers — the partitioned FSI work already had to bolt a
  branch onto run_simulation (main.cpp#L558) that bypasses
  iterate_solution entirely (partitioned_fsi->run() vs iterate_solution()).

As more schemes are added, this pushes scheme-specific control flow into main.cpp instead of behind a clean interface. This refactor is also a prerequisite for #554.

Solution

Replace the free iterate_solution with a polymorphic time-stepping abstraction — e.g. a virtual Simulation::run() (or a TimeIntegrator /strategy object) — where:

• the default implementation is today's loop (one nonlinear solve per step), preserving current behavior;
• subclasses / strategies override the per-step behavior — G&R load stepping, partitioned FSI coupling, etc.;
• main.cpp calls only the polymorphic entry, so no scheme-specific branching remains there (the run_simulation partitioned-FSI branch is subsumed).

This gives G&R a natural entry point for load steps and lets #554 plug in without touching main.cpp.

Additional context

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