Modelly 1.3.1

No score is slower than 1.2.0 any more: benchmarked against the 1.2.0 wheel with identical
dependencies (core operations, the playbooks and a research notebook, in paired alternating
runs; docs/performance.md, "No regression from release to release").

Performance

• Reading a Column attribute costs what it did in 1.2.0 again. 1.3.0's getter kept the
  column's name for the error of a removed agent, which cost about 2.5% per read; it now looks
  the name up only when the error is raised. The error is unchanged.
• On regular (GIL) builds, the neighbourhood walk (Cell.get_neighborhood) and the cell move
  are 1.2.0's code again, without the free-threading safety 1.3.0 added to every call (a
  reference count per neighbour, a cache-generation check per move). Free-threaded builds
  keep their safe paths.
• FixedAgent.remove reads its cell once.

Fixed

• A timing test of modelly profile --sizes compares populations 16 times apart, so it no
  longer fails on a slow runner.