Require JAX 0.6+ for import compatibility

[talolard]

Summary

Fixes #55 for the current main package line by aligning package metadata with the JAX API era that no longer exposes internals through jax.core.

Current main no longer contains the legacy jaxls/typed.py code path that referenced jax.core.JaxprEqn; a fresh import succeeds with JAX 0.6+. This PR makes that compatibility explicit by:

• raising the runtime dependency floor to jax>=0.6.0 and jaxlib>=0.6.0
• adding a subprocess import regression test so import-time failures from removed JAX internals are caught directly

Validation

• make format fails before running because Makefile references an undefined examples extra: error: Extra examples is not defined in the project's optional-dependencies table
• uvx ruff format .
• uvx ruff check .
• uv run --python 3.12 --with-editable . --with 'jax==0.6.0' --with 'jaxlib==0.6.0' python - <<'PY' ... import jax; import jaxls ... PY
• uv run --python 3.12 --extra dev --extra cholmod pytest
• uv run --python 3.12 --extra dev --extra docs --extra cholmod pyright .
• uv run --python 3.12 --extra dev python transpile_py310.py && git diff --quiet src/jaxls/_py310/
• uvx ruff check . && uvx ruff format --check .
• for py in 3.10 3.11 3.12 3.13; do uv run --python "$py" --extra dev --extra cholmod pytest || exit $?; done

Note: local cholmod validation required installing the same system dependency used by CI: libsuitesparse-dev.