Check layer locks contain wheels for all target platforms

[ncoghlan]

#254 switched to using the pylock.toml locking format, which records the details of which wheels corresponding to which hashes (unlike the previous flat requirements file format).

It should be feasible to check the generated lock files to confirm that wheels are available for all of the target platforms expected for a given layer specification. This is mainly a matter of checking for the relevant compatibility tags in the wheel filenames (ensuring to account for pure Python any wheels and macOS deployment target version details)

(uv itself won't be ensuring that is the case, as the attempt to do so in #306 resulted in lock and package summary files that contained too much repetitive environment marker noise to be usable)