Landing page v2: visual narrative (dots-forward stage + effect gap)

[acarril]

Summary

Evolves the landing-page hero from a single visual (M-distributions reweighting) into a dots-forward evolving stage that gives every narrative beat visual support, and fixes the scrub timing so balance lands at scene 04.

• Dots are now the primary actors: they start as two 2D clouds (scene 01), gather and align onto the M axis (scene 02), then resize as they reweight (scenes 02→04). Density curves fade in at scene 02 as the balance verdict.
• Live effect-gap (Δ) panel (the causal payoff): a compact τ-axis with two markers and the gap Δ, computed from a synthetic per-unit effect τ = α + β·G + γ·M. As M balances, the two markers diverge and Δ grows 4pp → 9pp (the Fujiwara "effect was masked by the confounder" story). Δ and std-diff recompute live from the same seed-42 IPW weights.
• Two-phase scroll timeline: align (scene 1→2) then t (scene 2→4). t now reaches 1 exactly at scene 04 (verified), with scene 05 holding — fixes the prior issue where the minimum landed at the bottom of scene 05.
• Scene 05 gains a faint hidden-dimension cue ("unobserved U") for the unconfoundedness caveat.
• Robustness preserved: prefers-reduced-motion renders the static balanced end-state; no-JS shows all scene text; mobile layout retained.

Docs-only (no R/Stata package code) -> no version bump, per the rescoped CLAUDE.md rule.

Spec: specs/2026-05-29-wsga-landing-v2-visual-narrative-design.md
Plan: specs/2026-05-29-wsga-landing-v2-visual-narrative-plan.md

Test plan

• node specs/verify_hero_math.mjs -> PASS (std-diff 0.94->0.04, effect gap 4.0pp->9.0pp)
• Headless-Chrome captures of all beats (setup / problem / reweight / balance / caveat) + reduced-motion end-state
• Timeline probe confirms t=1 at scene 04, held at scene 05
• Reviewer: open the deployed/local page and confirm scroll-scrub feel, effect-panel readability, and mobile layout
• After merge: confirm alvarocarril.com/wsga/ shows the v2 hero once Pages rebuilds