Uncertainty narrative reorg: measurement → depth → stress (tracking)

[mdenolle]

Epic / tracking issue for reorganizing the Quarto uncertainty pages into the forward-propagation arc measurement error → depth → stress/strain at depth.

Target: a 3-page "Uncertainty" pillar (theory-uq folded into the stress page, then deleted), plus an attribution page.

1. Measurement UQ → structured $C_d$ — UQ reorg · Page 1: Measurement UQ (merge + processing-choice sampling) #3
2. Depth UQ → $\delta V_S/V_S(z)$ posterior — UQ reorg · Page 2: Depth UQ (frequency → depth propagation) #4
3. Stress / effective stress / strain at depth (+ folds theory-uq), new uq_stress_depth module — UQ reorg · Page 3 + new module uq_stress_depth.py (stress/effective-stress/strain at depth) #5
4. Attribution of stress to forcing source, with uncertainty — UQ reorg · Page 4: Attribution of (effective) stress to forcing source, with uncertainty #7

• Nav / cross-links / delete old pages — UQ reorg · Nav, cross-links, delete old pages #6

See quarto/REORG_PLAN.md for full context (§1–10).