Build LINESTRING XYZM trips (integer M from a time column)

Minimal helper to create LINESTRING XYZM for mapdeck::add_trips() using an integer range from a time/index column (e.g. "dispersaltime").

type = "points": group points by group_col (default "id") ordered by time, build one LINESTRING per group with M = integer(time).
type = "linestring": if id_col present, collapse time-sliced features per id and ramp M linearly from t -> next_t along vertices; last slice uses next_t = t + 1. If no id_col, each feature ramps t -> t+1.

Usage

sf_to_xyzm(sf_obj, time_col = "dispersaltime", z_val = 0, cast_multi = TRUE)

sf_obj: sf object (POINTS for type="points"; LINESTRING/MULTILINESTRING for type="linestring")
time_col: character name of time/index column (integers or numeric)
z_val: numeric constant Z (default 0)
cast_multi: logical; cast MULTILINESTRING to LINESTRING (default TRUE)
type: c("points","linestring")
group_col: character, group/id column for points (default "id" if present)
id_col: character, id column for linestring collapse (default "id" if present)

Sets geometry m_range attribute required by mapdeck.