Split the Python runtime into cultcache-py, cultnet-py, cultmesh-py

One distribution named cultcache-py carried three libraries. Rust and
TypeScript layer them as cultcache < cultnet < cultmesh; Python now does
the same, one package per layer, module names unchanged so every
`from cultmesh_py import ...` keeps working.

Two imports ran against that layering and are resolved by moving code to
where the dependency direction says it lives, not by adding a shim:

- cultnet_py.interop_peer imported the Verse/peer catalog, authority
  lease, stream, and simulation-fact contracts from cultmesh_py. They are
  wire contracts a bare peer has to speak without owning a node, which is
  why cultnet-rs and cultnet-ts keep them in the net layer. They now live
  in cultnet_py.cultmesh_contracts; cultmesh_py re-exports them.
- cultcache_py.benchmark imported cultnet_py. It measures framed CultNet
  parse beside cache paths, so it and compare_csharp move to cultnet_py.
  verify checks all three packages and moves to cultmesh_py.

The single 6,693-line test module is split by what each test touches: a
test using any cultmesh_py name lands in cultmesh-py, else cultnet_py in
cultnet-py, else cultcache-py. Each suite runs with only its layer and the
layers below on the path, so a test that reaches up fails to import
rather than quietly binding the packages back together. 11 / 53 / 78 =
the same 142.

Workflows install all three; the publish job is one matrix over the
three packages keyed by tag prefix. The TS test harnesses put all three
src directories on PYTHONPATH. cultcache-py bumps to 0.2.0 because it no
longer contains cultnet_py or cultmesh_py; the new packages start at
0.1.0. Nothing was ever published, so no PyPI consumer moves.

  cultcache-py  11 tests, path: cultcache-py/src
  cultnet-py    53 tests, path: + cultnet-py/src
  cultmesh-py   78 tests, path: + cultmesh-py/src
  python -m cultmesh_py.verify -> ok, all five checks
  cultnet_py.interop_peer imports with cultmesh_py absent

Not run here: the cultnet-ts interop ring and cultcache-ts store matrix
need dotnet/kotlin peers; their PYTHONPATH change is four lines.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

.gitignore

@@ -303,6 +303,10 @@ node_modules/
 !packages/cultmesh-ts/**
 !packages/cultcache-py/
 !packages/cultcache-py/**
+!packages/cultnet-py/
+!packages/cultnet-py/**
+!packages/cultmesh-py/
+!packages/cultmesh-py/**
 !packages/cultcache-rs/
 !packages/cultcache-rs/**
 !packages/cultnet-rs/