[lawrennd]

Summary

• Replace missing includes mocap-ppca.md and pca-in-practice.md with osu-run1-ppca.md and pca-effectiveness.md.
• Keep ppca-interpretations.md (it already nests PCA, matrix factorisation, and model/algorithm). Add a Derivation of PPCA section before the marginal-likelihood maths.
• British spelling in the review notes (parameters, maximising, minimising, marginalise).
• Source only. Rebuild 09-dimensionality-reduction.ipynb after merge to avoid conflicting with Add notebook smoke tests and rebuilt lectures #2.

Test plan

• After merge, rebuild week 9 from _lamd/ and confirm OSU run PPCA and PCA effectiveness compile.
• Confirm PCA is not duplicated: the second include inside interpretations is a no-op under \ifndef.