Control-region closure + control-plot transforms, SR cut

[ludovicomori]

Postprocessing / control plots:

• singleVarHist: optional transform callable applied to raw values before filling (axis must already span the transformed range).
• control_plots: display flagged discriminator-score vars under x -> -log(1 - x + eps) (TRANSFORM_VARS / TRANSFORM_EPS), stretching the pile-up near 1. ratioHistPlot just renders the hist's axis, so the transform is done at fill time.
• --sr flag for the --control-plots pipeline: apply the signal-region pass cut (resolved from --bmin / --sensitivity-dir via Regions.get_selection_regions) before plotting. Control-region closure (new control_region_closure.py):
• Closure test for the data-driven QCD+DY estimate using the --control-region templates; now renders each MC background as its own stacked sample (reusing plotting.ratioHistPlot) plus the data-driven QCD+DY and a data/pred ratio.
• Includes the --control-region template plumbing it depends on (control_region kwarg through main/get_templates/parse_args, "control" template subdir). Notebook fix:
• bdt_utils only forces the Agg backend when an interactive (module://...) backend is not already active, so importing it from a notebook no longer silently breaks plt.show(). Cleanup:
• Move personal scratch (notes/, paper helpers, inspector.ipynb, etc.) into untracked_utils/ and gitignore it.

[ludovicomori]

WARNING this PR is outdated and its commits are part of #95. Please review #95 directly.