Expand sampling baseline literature review

[MaxGhenis]

Summary

• broaden the related-work framing to cover raking before/after sampling, integerisation/TRS, balanced sampling, combinatorial optimisation, and convex sparse calibration
• clarify which methods are relevant literature but not direct full-surface baselines for the mixed Populace target surface
• add bibliography entries for IPU/population synthesis and the cube method

Verification

• quarto render paper/index.qmd
• citation key check
• git diff --check

[juaristi22]

Sorry for the conflict, I just added the CI fix to paper-results. Let me know if you'd like me to revert it.