dft_stac_composite(aggregation = "count") silently returns reflectance, not a clear-observation count

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Problem

dft_stac_composite(aggregation = "count") returns reflectance, not a count. It raises no error and no warning.

aggregation goes straight into gdalcubes::cube_view(aggregation = ) (R/dft_stac_cube.R, stac_cube_assemble), which supports only "min", "max", "mean", "median" and "first" (gdalcubes 0.7.5, ?cube_view). An unsupported value is not refused. What comes back looks like surface reflectance.

This was found by floodplains#93 phase 2. That phase measures the clear-observation windows for reference imagery exactly as drift's docs describe: aggregation = "count", bands = "red", with the SCL mask applied before aggregation.

Repro (drift 0.19.0, gdalcubes 0.7.5)

A 2 km square in the NECR floodplain, July, res = 20:

r <- dft_stac_composite(aoi, years = 2021, months = 7, bands = "red",
                        aggregation = "count", res = 20, crs = "EPSG:32610")[[1]]
summary(terra::values(r))
#> Min 0.0077  Median 0.0331  Max 0.1835          # 2021
#> Min 0.0078  Median 0.0411  Max 0.1802  NA 307  # same call, 2023

The values are continuous and sit at typical vegetation red reflectance. The per-pixel maximum for the month should be at most the 17 items the query returned (6 distinct dates). The values are not integers times the band scale (1e-4) either, and 2023 carries no −0.1 offset shift. So this is not a count that was then scaled. "count" is simply not being honoured.

Asks

1. Validate aggregation against what cube_view supports, in dft_stac_composite() and dft_stac_cube(), and refuse anything else. A wrong value should never return plausible reflectance.
2. Provide clear-observation counts per window. The obvious way is dt = "P1D" in the view, then gdalcubes::reduce_time(cube, "count(<band>)") over the window, with no scale or offset applied to the result. Either a new aggregation = "count" path or a separate function. The count is what a caller needs to choose composite windows, and the composite docs already suggest it.

Also affected

floodplains#93's issue body tells callers to use aggregation = "count". That call has to wait for this issue.
3. Invalidate cached "count" composites. The cache key hashes aggregation, so every composite_<key>.tif written by 0.19.0 with aggregation = "count" holds reflectance under the key a fixed count would reuse. The fix has to change the key, for example by versioning it, or those stale files will be read back as counts.
4. Count acquisitions, not items. Where adjacent MGRS tiles overlap, one acquisition appears as two items. The validation square above spans 3 tiles (09UYV, 10UCE, 10UDE): 17 items but only 6 distinct dates. A count should de-duplicate same-day scenes (dt = "P1D" does this if the day is the time step). Also note that snow classes are in the default mask_values, so a spring or autumn "clear" count excludes snow as well as cloud. The docs should say so.