frs_wsg_drainage: FWA WSG drainage-closure primitive

[NewGraphEnvironment]

Problem

link currently computes WSG drainage closure (every WSG a focal set drains through, downstream-first) inline in data-raw/study_area_wsgs.R — see NewGraphEnvironment/link@v0.40.5. The query reads public.wsg_outlet, joins focal → closure via f.outlet <@ w.outlet, sorts by nlevel(outlet) ASC.

This is FWA-topology — it belongs in fresh, not link. No bundle/species/overrides knowledge involved; pure network shape.

Proposed

frs_wsg_drainage(conn, wsgs)

• wsgs: character vector of WSG codes (the focal set)
• Returns: character vector covering the drainage closure (focal + every WSG they flow through), ordered downstream-first (nlevel(outlet) ASC)
• Pure FWA topology — no bundle / species / overrides logic

Where it lives

R/frs_wsg_drainage.R. Sits alongside frs_wsg_species() as part of a frs_wsg_* family — the WSG-topology surface of fresh.

Why fresh, not link

wsg_outlet is FWA infrastructure. Drainage closure is a network-shape question with no link/bundle knowledge. Other fresh consumers (vignettes, ad-hoc analyses, future provincial drivers in other packages) want this without going through link.

Acceptance

• frs_wsg_drainage(conn, c("PARS","BULK")) returns the 15-WSG Skeena+Peace closure: KISP, KLUM, LKEL, LSKE, MSKE, USKE, BULK, FINA, LBTN, LPCE, MORR, PARA, PCEA, UPCE, PARS — matches NewGraphEnvironment/link@v0.40.5 study_area_wsgs.R output
• DS-first ordering preserved
• Closure invariant to focal ordering
• Runnable @example
• Test against a small live focal set

Replaces

Inline closure query in NewGraphEnvironment/link@v0.40.5 data-raw/study_area_wsgs.R (lines computing wsg_outlet-based closure).

Composes with

link::lnk_wsg_resolve (NewGraphEnvironment/link#207) — the link wrapper adds the bundle's species-presence filter.

Naming considered

frs_wsg_drainage (chosen) is frs_wsg_*-family consistent. wsg is FWA-specific terminology — honest to fresh's current FWA-only scope. If fresh ever grows a second network (NHD/HUC), a network = "fwa" param can be introduced then (YAGNI today).