Repo - Finish the Fortran 1.6.0 parity checks left open at the 2.0.0 release #640

Four checks from the Fortran GPEC 1.6.0 parity audit (#591) were left open when the parity series (#625, #626, #627) closed it. None is a known bug; each is a trace that was not finished.

• SLAYER radial normalisation (Fortran PR 249). Confirm that _rs_at returns ρ_tor and _da_dpsi_at returns dρ_tor/dψ_N in src/InnerLayer/SLAYER/LayerInputs.jl. The Fortran bug passed the derivative where r_s was expected.
• qa = qlim (Fortran PRs 225 and 234). The free.f change that sets qa = qlim was not traced into the Julia free-boundary path.
• Direct equilibrium ψ bounds (Fortran PRs 298 and 335). The ldp_i and grid_type = "original" bounds were checked in ReadEquilibrium.jl and InverseEquilibrium.jl; the direct path was not inspected.
• pmodb gauges docs (Fortran PR 339). Mirror the gauge statements in docs/src/conventions.md.

The Bernstein plasma-energy breakdown (#632, closed in favour of #465) is tracked on #465.