Tighten vignette citation prose: surface specific findings, drop decorative attribution

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Problem

The 8 new citations added in v0.2.5 (#65/#66) and v0.2.6 (#67/#68)
to the regional vignettes mostly land at the end of factual
statements as parenthetical attributions. Reads like name-dropping
— the cites identify the source but don't surface what those papers
actually found. Reader has to look up each paper to know what's
being grounded by the citation.

Scope

Tighten prose around 3 cite locations in both regional vignettes
(kootenay-lake.Rmd, peace-fwcp.Rmd) so the source's specific
findings show up in the prose. Drop 1 cite (Arguez & Vose 2011)
that was decorative-only attribution.

Cite	Action
hansen_etal2012Perceptionclimate	Keep — already does work
arguez_vose2011DefinitionStandard	Cut — decorative; WMO standard is well-known fact
karl_etal1993NewPerspective	Tighten — add 3:1 ratio + 0.84/0.28 °C numbers
pepin_etal2015Elevationdependentwarming + rangwala_miller2012Climatechange	Keep — dual-cite affirmation + caveat already does work
ficklin_novick2017Historicprojected	Keep — post-agent-fix already names mechanism + scope
mantua_etal2010Climatechange + eaton_scheller1996Effectsclimate	Tighten — surface WA-watersheds finding + 57-species framing
kang_etal2016ImpactsRapidly	Tighten — add ~10-day Fraser advance + salmon-migration-window detail

After this, vignettes/references.bib will go from 18 → 17 entries
(Arguez & Vose dropped). Run Rscript data-raw/regenerate_bib.R
post-edits per the v0.2.7 workflow.

Authority audit

All 8 cites were confirmed legitimate authorities for the specific
claims they're attached to (per the pre-work audit). No removals on
authority grounds — only on prose-style grounds.

Done when

• 4 prose edits applied per vignette (8 edits total): Arguez
  removal, Karl tightening, Mantua + Eaton tightening, Kang
  tightening
• Same proposed wording applied to both vignettes (they share
  the cite set)
• Audit log update notes the tightening + drops Arguez row
• Independent review agent verifies the new tightened prose is
  faithful to the cited sources (numbers right, geographic
  scope right, no new overreach)
• vignettes/references.bib regenerated; should drop to 17
  entries
• Vignette renders cleanly (local + CI pkgdown)
• PR + release v0.2.8 (patch — vignette docs)