Dangling citation keys @lobkis01 and @poupinet84 have no matching bib entry

[mdenolle]

Found during a full quarto/latexmk build QA pass while resolving the marine-review backlog (PR #29).

paper/manuscript_marine.qmd around lines 421-422 (methods appendix, describing the stretching and windowed-cross-correlation estimators):

trace stretching (TS, @lobkis01), windowed cross-correlation (WCC, @poupinet84)

Neither lobkis01 nor poupinet84 exists in references.bib or survey.bib. latexmk confirms these are the only two remaining unresolved citations after this PR's other fixes.

Likely real candidates (not verified, do not cite without checking):

• WCC (doublet method): Poupinet, G., Ellsworth, W. L., & Fréchet, J. (1984), "Monitoring velocity variations in the crust using earthquake doublets: An application to the Calaveras Fault, California," JGR, 89(B7), 5719-5731.
• TS (stretching): possibly Lobkis & Weaver (2003), PRL, on coda-wave interferometry, or the technique may already be adequately covered by @SensSchonfelder2006 (already in references.bib) -- needs a literature check to confirm which is the intended source.

Acceptance criterion: verify and add the correct bib entries (or reuse an existing key if it already covers the claim), then update the citations in the manuscript.