#122 Add transportability and external validity lecture

Continues the assumption-focused causal-inference layer.

What changed

• adds 04-causal-inference/transportability-external-validity/;
• adds the canonical presentation/main.tex entry point;
• adds Transportability and External Validity;
• covers internal vs external validity, source and target populations, effect modification, standardization, transport weighting, support, covariate shift, site heterogeneity, trial-to-target generalization, temporal drift, treatment-version differences, and sensitivity to unmeasured effect modifiers;
• registers the deck in the presentation contract validator;
• registers the deck in GitHub Actions and compile_all.sh;
• updates the causal-inference domain index.

Review note

This PR intentionally does not merge automatically.

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Summary by cubic

Adds a new lecture deck on transportability and external validity to the causal-inference layer, covering when causal effects estimated in one population, site, or time period can be applied to another.

This PR intentionally does not merge automatically.

• Adds the 04-causal-inference/transportability-external-validity/ topic with README, main.tex entry point, and the 33-slide beamer lecture.
• Registers the deck in GitHub Actions, compile_all.sh, the presentation contract validator, and the causal-inference domain index.