`$classes` attribute can be outdated

[DavisVaughan]

I noticed that in some cases the classes attribute that is on a sfc_GEOMETRY column can be outdated in some cases. In particular, when [<- is used and the sfc_GEOMETRY becomes a uniform sfc_POINT due to the assignment, the classes attribute isn't updated (ideally in this case I think it should be removed? Because the vector is a uniform geometry now?)

library(sf)
#> Linking to GEOS 3.13.0, GDAL 3.8.5, PROJ 9.5.1; sf_use_s2() is TRUE

# Mixed geometries
x <- st_sfc(st_point(), st_linestring())
x
#> Geometry set for 2 features  (with 2 geometries empty)
#> Geometry type: GEOMETRY
#> Dimension:     XY
#> Bounding box:  xmin: NA ymin: NA xmax: NA ymax: NA
#> CRS:           NA
#> POINT EMPTY
#> LINESTRING EMPTY

# Abstract `sfc_GEOMETRY`
class(x)
#> [1] "sfc_GEOMETRY" "sfc"

# Note that `classes` tracks this
attributes(x)$classes
#> [1] "POINT"      "LINESTRING"

# Make `x` a uniform vector of sfc_POINTs
x[2] <- st_point()

# Looks like the overall geometry type updated
x
#> Geometry set for 2 features  (with 2 geometries empty)
#> Geometry type: POINT
#> Dimension:     XY
#> Bounding box:  xmin: NA ymin: NA xmax: NA ymax: NA
#> CRS:           NA
#> POINT EMPTY
#> POINT EMPTY

# But `classes` hasn't been updated
attributes(x)$classes
#> [1] "POINT"      "LINESTRING"

# Ideally I think `classes` would be totally removed here,
# since `x` is now uniform and `classes` is not required.

^{Created on 2026-01-05 with reprex v2.1.1}