Deep SCM estimation stress

Sibling to the main stress validation notebook: synthetic bump spectra → RepresentationSpec codes → mediation / LMTP / missing Y.

Quarto: docs/stress/deep_scm_estimation_stress.qmd

cd docs/stress && quarto render deep_scm_estimation_stress.qmd

Mechanisms and L3 live in CausalDynamics Deep SCM stress. Full MIRS / Twins cohort ingestion remains an open gap (causal-dynamics-book#14).