Date: 2026-08-09 Purpose: Documentation that spans the fem submodules (kernel, iwg, maxwell, ...). Submodule-specific documentation lives in src/fem/<submodule>/doc/.

Document	Content
timestepping_strategy.md	Time integration schemes (BDF1–5, ratio clamps, startup ramp), the PID step-size controller and its legacy fallback, failure/floor policy, warm-start persistence of Δt AND the BDF order/step-size history (2026-08-15), literature (PI/PID step control)

src/fem/kernel/doc/nonlinear_controller_theory.md — the inner nonlinear loop (Picard/Newton hybrid, relaxation adaptation, guards)
doc/input_file_reference.md — deck keys