Harden fl_valley_confine() against XYZM streams from fwapg

[NewGraphEnvironment]

Problem

fl_valley_confine() calls sf::st_buffer() internally for the channel_buffer feature (R/fl_valley_confine.R:202). When streams are pulled from fwapg (e.g., via fresh::frs_stream_fetch()), the geometries are XYZM — route measures live in M. GEOS rejects XYZM:

Error: GEOS does not support XYM or XYZM geometries; use st_zm() to drop M

Every caller pulling from fwapg has to remember to sf::st_zm(streams) before calling fl_valley_confine(). Surfaced during #34 work; for now the data-raw script (data-raw/wsg_vignette_data.R) has an explicit sf::st_zm() workaround.

fl_dem_aoi() was hardened the same way during #34 (drops Z/M internally before sf::st_buffer()).

Proposed Solution

One-line fix in fl_valley_confine(): drop Z/M at the top of the function once streams is identified as sf. Mirrors what fl_dem_aoi() does.

if (inherits(streams, "sf")) {
  streams <- sf::st_zm(streams)
  ...
}

Also worth a test: pass an XYZM streams sf and assert it runs end-to-end without erroring.

References

• Worked around in flooded Add DEM source helpers (file, STAC) for AOI-driven workflows #34 via sf::st_zm() in data-raw script
• Same fix shipped in fl_dem_aoi() during Add DEM source helpers (file, STAC) for AOI-driven workflows #34