Post-#118 follow-up: merge gompertz scripts + update bias-figure TODO

[adibender]

Summary

Follow-up to PR #118 (intro restructure, merged 2026-05-01) that picks up two small post-merge cleanups stranded on the intro-draft3 branch.

Changes

• Merge intro_gompertz_human.R into code.R: The Swedish-mortality Gompertz fit (a = 2.88e-4, b = 0.097 on 2019 ages 30-100, fitted via population-weighted log-linear regression on eha::swedeaths / eha::swepop) replaces the older synthetic dstr("Gompertz", shape = 2) block at the top of code.R. Both scripts wrote to the same output path book/Figures/introduction/gompertz.png so the duplication was a stale-output hazard. Tested locally — produces a clean four-panel figure (density, hazard, CDF, survival, ages 30–110).
• Delete the standalone book/experiments/intro_gompertz_human.R since its content is now in code.R.
• Add library(eha) to the top-level package loads in code.R (needed by the new section).
• Update bias-figure TODO in book/P0C1_intro.qmd:136 from a content-suggestion ("remove left-truncation, delayed entry and immortal time") to a hand-off note: <!-- TODO (Raphael): restyle figure for visual consistency; keep all 5 rows -->. This reflects the agreed direction in the Mainly chapters 1-3, with some other reviewer comments thrown in sorry #118 review thread (Raphael preferred keeping all 5 rows but restyling).

Notes

• This is a clean cherry-pick of commit 3867da0 from the dormant intro-draft3 branch.
• The figure-removal commit (a63a1b3 on intro-draft3) was deliberately not cherry-picked: PR Add P1C7: Start-to-finish ML survival analysis chapter #175 (p1c7-start-to-finish) introduces the new chapter that uses the ml-sa-pipeline.png figure, but that PR doesn't currently re-add the figure (it was already tracked from Mainly chapters 1-3, with some other reviewer comments thrown in sorry #118). Removing it on main here would leave PR Add P1C7: Start-to-finish ML survival analysis chapter #175's chapter referencing a missing figure. The figures stay on main as orphan-but-harmless until Add P1C7: Start-to-finish ML survival analysis chapter #175 lands.

Test plan

• Render the relevant section by sourcing the Gompertz block of code.R standalone — done locally, no warnings.
• Render book/P0C1_intro.qmd to verify TODO comment doesn't break anything (it's just a comment).