Maintenance and minor feature release

Maintenance

• Add support and wheels for Python 3.15
• Require Python >= 3.12

Features

• CLI commands that load a tree sequence now read from stdin when the input path argument is omitted.
• The returned object from variant.counts() and variant.frequencies() now stores alleles in the order defined in variant.alleles.
• If top-level metadata is large, then repeatedly accessing it can be costly, so now it throws a warning if the metadata is more than 200Kb and it is accessed more than 20 times alerting the user to this possibility (both constants are modifiable, however).
• TreeSequences and TableCollections now have a metadata_size property, returning their size in bytes.

Bugfixes

• Update to kastore C version 2.1.3 to include file security fix
• TableCollection.delete_older now checks table integrity first, raising an error for invalid node or mutation references instead of reading out of bounds