Fix 5 forward references among definitions and results #1389

The dependency graph script (gha#1126) found 0 cycles and 5 forward references: a div that uses one defined later on the same page.

• Linear-models-overview#def-independence-diagnostics uses def-breusch-godfrey-diagnostic and def-durbin-watson-diagnostic
• causal-inference#def-confounding-obs uses def-confounder
• causal-inference#def-exchangeability uses def-confounding-obs
• intro-to-survival-analysis#thm-kappa-complement uses def-discrete-haz-risk

Fix: move each used definition before its first use, or split the using div.

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https://claude.ai/code/session_01DbF799uMdieeoBVQFaapJg