Vignette: WSG x ecoregion overlap table with curated commentary

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Problem

The peace-fwcp vignette currently shows two orthogonal partitions of the FWCP Peace Region — 18 watershed groups (WSGs) and 5 ecoregions — but doesn't connect them. For users planning at the WSG scale (the FWCP funding unit, the fish passage reporting unit), the question is: "Which ecoregion does my watershed group sit in, and does the climate departure story for that ecoregion apply to my work?"

This matters because the per-ecoregion story is not uniform — Boreal Mountains and Plateaus and Northern Canadian Rocky Mountains show statistically significant precipitation increases; the southern and central ecoregions do not. WSGs spanning a boundary will see a different climate-departure narrative than WSGs that sit entirely within one ecoregion.

The vignette will become a template for three reporting climate-departure appendices and will port directly to the upcoming fish_passage_peace_reporting_2025 report. WSG ↔ ecoregion linkage is the bridge between the analytical units (ecoregions) and the reporting units (WSGs).

Proposed Solution

Add a small WSG × ecoregion overlap section to the vignette, after Per-Ecoregion Variation and before Interpretation:

1. Computed table — for each of the 18 WSGs, the percent of its area falling in each of the 5 ecoregions. Wide format: WSG code | OMM% | BMP% | CRM% | FAB% | NRM% | total km².
2. Curated commentary column — one short plain-language note per WSG. Hand-written once, shipped as inst/extdata/peace_wsg_ecoregion_commentary.csv. Examples:
   • "Entirely within Omineca Mountains."
   • "Split ~60/40 between Boreal Mountains & Plateaus and Northern Canadian Rockies, west-east axis."
   • "Largely Central Canadian Rockies with a Northern Rockies headwater fringe."
3. Interpretation tie-in — one paragraph noting which WSGs sit in the precipitation-trending-up ecoregions versus the no-trend ones.

Why this scope

• All polygons are already bundled in inst/extdata/context_fwcp_peace.gpkg, so no new data fetching.
• Auto-generating the commentary from rules ("primarily east-west", "headwater fringe") is brittle — the geographic nuance is small and worth writing 18 lines by hand once.
• Ports general — same WSG × ecoregion overlap pattern works for any BC region. If we generalize, this could become a reusable helper inside cd for any AOI-based report.

Out of scope

• General cd_overlap() helper function — could come later if useful, but the vignette table is enough for now.
• Per-WSG climate departure (different question — would need 18 × cd_extract calls; probably not worth it given the regional-uniformity finding for temperature).