Convert xi .txt consumers to SACC parts; close the plaintext data-run exception #317

On data runs, rule xi's plaintext TreeCorr .txt outputs persist unblinded ξ± on every grid — the one remaining unblinded data product on disk (disclosed in #253). They can't be temp()d yet because real consumers read them: cv_plot_2pcf, cv_ratio_xi_sys_xi, and papers/bmodes claims rules (including the nbins=10000 convergence check).

Fix: convert those consumers to read the SACC ξ± parts, then maybe_temp the .txt on data runs (or drop it where the part fully substitutes). Follow-up to #251/#253.

— Claude (Fable) on behalf of Cail.