Map a methodology mode to its short-code (M1 / M2 / M3)
Source:R/output_stamping.R
methodology_short_code.RdUsed in run-directory names and filenames where the full mode string
would be visually noisy. The mapping is fixed: reflexive_scaffold = M1,
codebook_collaborative = M2, framework_applied = M3. Returns
"M?" for unknown / NULL modes (visible-failure rather than
silent-empty-suffix).