Make stability analysis more robust - #405

State::stability_analysis:

• Failed trial phases no longer abort the analysis. A trial whose tpd minimization fails is skipped, or kept if its tpd is already negative, since that proves instability. Errors are returned only if no trial phase could be evaluated.
• Candidates are sorted by their true tpd, most negative first. tp_flash now initializes from the first two candidates instead of the last two in trial order.
• Duplicate candidates are removed. Partial densities are compared relative to the total density, so the same phase found from two trials, differing only in trace components, is reported once.
• Fix NaN tpd for absent components (0 ln 0). This also affected the convergence check in tp_flash.
• Moved tangent_plane_distance to stability_analysis.rs.

Tests cover failed trials, sorting, duplicates and zero components.