Summary

This PR improves the sawteeth source modeling and makes related API cleanups.

Sawteeth Source Improvements

• Configurable flattening: Expose flat_factor parameter (default 0.5) to control the degree of profile flattening inside the inversion radius, rather than using a hardcoded value
• Better inversion radius detection: Use dd.sawteeth.diagnostics.rho_tor_norm_inversion when available, falling back to q-profile based detection
• Correct source exclusions: When computing the total source to flatten, exclude intrinsic sources, which should not be flattened because they are computed from the kinetic profiles themselves (e.g., bootstrap current should not be flattened).
• Include sawteeth in j_non_inductive: The sawteeth source represents redistribution of other non-inductive sources within the mixing radius, so it should be included in j_non_inductive (previously excluded)

Intrinsic Sources Changes

• Renamed sources! → intrinsic_sources! (BREAKING) : More accurately describes that this function calculates intrinsic sources (collisional exchange, bremsstrahlung, fusion, bootstrap, ohmic) as opposed to externally-driven sources
• Reordered calculations: Fast ion densities are now updated before bootstrap (which depends on them) for consistency

Plotting Fix

• Sources with large magnitudes but near-zero integrals (like sawteeth, which redistribute rather than add energy) are now correctly displayed by using the integral of absolute values for the visibility threshold

Companion to ProjectTorreyPines/FUSE.jl#1045